Media over QUIC Transport
draft-ietf-moq-transport-11
| Document | Type | Active Internet-Draft (moq WG) | |
|---|---|---|---|
| Authors | Suhas Nandakumar , Victor Vasiliev , Ian Swett , Alan Frindell | ||
| Last updated | 2025-04-28 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-moq-transport-11
Media Over QUIC S. Nandakumar
Internet-Draft Cisco
Intended status: Standards Track V. Vasiliev
Expires: 30 October 2025 I. Swett, Ed.
Google
A. Frindell, Ed.
Meta
28 April 2025
Media over QUIC Transport
draft-ietf-moq-transport-11
Abstract
This document defines the core behavior for Media over QUIC Transport
(MOQT), a media transport protocol designed to operate over QUIC and
WebTransport, which have similar functionality. MOQT allows a
producer of media to publish data and have it consumed via
subscription by a multiplicity of endpoints. It supports
intermediate content distribution networks and is designed for high
scale and low latency distribution.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://moq-
wg.github.io/moq-transport/draft-ietf-moq-transport.html. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-moq-transport/.
Discussion of this document takes place on the Media Over QUIC
Working Group mailing list (mailto:moq@ietf.org), which is archived
at https://mailarchive.ietf.org/arch/browse/moq/. Subscribe at
https://www.ietf.org/mailman/listinfo/moq/.
Source for this draft and an issue tracker can be found at
https://github.com/moq-wg/moq-transport.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.1. Latency . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2. Leveraging QUIC . . . . . . . . . . . . . . . . . . . 6
1.1.3. Convergence . . . . . . . . . . . . . . . . . . . . . 6
1.1.4. Relays . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Terms and Definitions . . . . . . . . . . . . . . . . . . 6
1.3. Notational Conventions . . . . . . . . . . . . . . . . . 7
1.3.1. Location Structure . . . . . . . . . . . . . . . . . 8
1.3.2. Key-Value-Pair Structure . . . . . . . . . . . . . . 8
1.3.3. Reason Phrase Structure . . . . . . . . . . . . . . . 9
2. Object Data Model . . . . . . . . . . . . . . . . . . . . . . 10
2.1. Objects . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2. Subgroups . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3. Groups . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.1. Group Ordering . . . . . . . . . . . . . . . . . . . 12
2.4. Track . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4.1. Track Naming . . . . . . . . . . . . . . . . . . . . 13
2.4.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . 13
3. Sessions . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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3.1. Session establishment . . . . . . . . . . . . . . . . . . 14
3.1.1. WebTransport . . . . . . . . . . . . . . . . . . . . 14
3.1.2. QUIC . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.3. Connection URL . . . . . . . . . . . . . . . . . . . 15
3.2. Version and Extension Negotiation . . . . . . . . . . . . 15
3.3. Session initialization . . . . . . . . . . . . . . . . . 16
3.4. Termination . . . . . . . . . . . . . . . . . . . . . . . 16
3.5. Migration . . . . . . . . . . . . . . . . . . . . . . . . 19
3.6. Congestion Control . . . . . . . . . . . . . . . . . . . 19
3.6.1. Bufferbloat . . . . . . . . . . . . . . . . . . . . . 20
3.6.2. Application-Limited . . . . . . . . . . . . . . . . . 20
3.6.3. Consistent Throughput . . . . . . . . . . . . . . . . 20
4. Retrieving Tracks with Subscribe and Fetch . . . . . . . . . 20
5. Namespace Discovery . . . . . . . . . . . . . . . . . . . . . 21
5.1. Subscribing to Announcements . . . . . . . . . . . . . . 22
5.2. Announcements . . . . . . . . . . . . . . . . . . . . . . 22
6. Priorities . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 23
6.2. Scheduling Algorithm . . . . . . . . . . . . . . . . . . 24
6.3. Considerations for Setting Priorities . . . . . . . . . . 25
7. Relays . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.1. Caching Relays . . . . . . . . . . . . . . . . . . . . . 26
7.2. Subscriber Interactions . . . . . . . . . . . . . . . . . 27
7.2.1. Graceful Subscriber Relay Switchover . . . . . . . . 27
7.3. Publisher Interactions . . . . . . . . . . . . . . . . . 28
7.3.1. Graceful Publisher Network Switchover . . . . . . . . 29
7.3.2. Graceful Publisher Relay Switchover . . . . . . . . . 29
7.4. Relay Object Handling . . . . . . . . . . . . . . . . . . 30
8. Control Messages . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Request ID . . . . . . . . . . . . . . . . . . . . . . . 32
8.2. Parameters . . . . . . . . . . . . . . . . . . . . . . . 32
8.2.1. Version Specific Parameters . . . . . . . . . . . . . 33
8.3. CLIENT_SETUP and SERVER_SETUP . . . . . . . . . . . . . . 37
8.3.1. Versions . . . . . . . . . . . . . . . . . . . . . . 38
8.3.2. Setup Parameters . . . . . . . . . . . . . . . . . . 38
8.4. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . . 39
8.5. MAX_REQUEST_ID . . . . . . . . . . . . . . . . . . . . . 40
8.6. REQUESTS_BLOCKED . . . . . . . . . . . . . . . . . . . . 40
8.7. SUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . . 41
8.8. SUBSCRIBE_OK . . . . . . . . . . . . . . . . . . . . . . 44
8.9. SUBSCRIBE_ERROR . . . . . . . . . . . . . . . . . . . . . 45
8.10. SUBSCRIBE_UPDATE . . . . . . . . . . . . . . . . . . . . 47
8.11. UNSUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . 48
8.12. SUBSCRIBE_DONE . . . . . . . . . . . . . . . . . . . . . 49
8.13. FETCH . . . . . . . . . . . . . . . . . . . . . . . . . . 52
8.13.1. Calculating the Range of a Relative Joining Fetch . 55
8.13.2. Calculating the Range of an Absolute Joining
Fetch . . . . . . . . . . . . . . . . . . . . . . . . 55
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8.14. FETCH_OK . . . . . . . . . . . . . . . . . . . . . . . . 56
8.15. FETCH_ERROR . . . . . . . . . . . . . . . . . . . . . . . 56
8.16. FETCH_CANCEL . . . . . . . . . . . . . . . . . . . . . . 58
8.17. TRACK_STATUS_REQUEST . . . . . . . . . . . . . . . . . . 59
8.18. TRACK_STATUS . . . . . . . . . . . . . . . . . . . . . . 59
8.19. ANNOUNCE . . . . . . . . . . . . . . . . . . . . . . . . 61
8.20. ANNOUNCE_OK . . . . . . . . . . . . . . . . . . . . . . . 61
8.21. ANNOUNCE_ERROR . . . . . . . . . . . . . . . . . . . . . 61
8.22. UNANNOUNCE . . . . . . . . . . . . . . . . . . . . . . . 63
8.23. ANNOUNCE_CANCEL . . . . . . . . . . . . . . . . . . . . . 63
8.24. SUBSCRIBE_ANNOUNCES . . . . . . . . . . . . . . . . . . . 64
8.25. SUBSCRIBE_ANNOUNCES_OK . . . . . . . . . . . . . . . . . 65
8.26. SUBSCRIBE_ANNOUNCES_ERROR . . . . . . . . . . . . . . . . 65
8.27. UNSUBSCRIBE_ANNOUNCES . . . . . . . . . . . . . . . . . . 67
9. Data Streams and Datagrams . . . . . . . . . . . . . . . . . 67
9.1. Objects . . . . . . . . . . . . . . . . . . . . . . . . . 68
9.1.1. Canonical Object Properties . . . . . . . . . . . . . 69
9.2. Object Datagram . . . . . . . . . . . . . . . . . . . . . 71
9.3. Object Datagram Status . . . . . . . . . . . . . . . . . 71
9.4. Streams . . . . . . . . . . . . . . . . . . . . . . . . . 72
9.4.1. Stream Cancellation . . . . . . . . . . . . . . . . . 72
9.4.2. Subgroup Header . . . . . . . . . . . . . . . . . . . 72
9.4.3. Closing Subgroup Streams . . . . . . . . . . . . . . 74
9.4.4. Fetch Header . . . . . . . . . . . . . . . . . . . . 77
9.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 78
10. Extension Headers . . . . . . . . . . . . . . . . . . . . . . 79
10.1. Prior Group ID Gap . . . . . . . . . . . . . . . . . . . 79
11. Security Considerations . . . . . . . . . . . . . . . . . . . 80
11.1. Resource Exhaustion . . . . . . . . . . . . . . . . . . 80
11.2. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . 80
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 80
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 81
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Normative References . . . . . . . . . . . . . . . . . . . . . 82
Informative References . . . . . . . . . . . . . . . . . . . . 83
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 84
A.1. Since draft-ietf-moq-transport-10 . . . . . . . . . . . . 84
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 85
1. Introduction
Media Over QUIC Transport (MOQT) is a protocol that is optimized for
the QUIC protocol [QUIC], either directly or via WebTransport
[WebTransport], for the dissemination of media. MOQT utilizes a
publish/subscribe workflow in which producers of media publish data
in response to subscription requests from a multiplicity of
endpoints. MOQT supports wide range of use-cases with different
resiliency and latency (live, interactive) needs without compromising
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the scalability and cost effectiveness associated with content
delivery networks.
MOQT is a generic protocol designed to work in concert with multiple
MoQ Streaming Formats. These MoQ Streaming Formats define how
content is encoded, packaged, and mapped to MOQT objects, along with
policies for discovery and subscription.
* Section 2 describes the data model employed by MOQT.
* Section 3 covers aspects of setting up a MOQT session.
* Section 6 covers mechanisms for prioritizing subscriptions.
* Section 7 covers behavior at the relay entities.
* Section 8 covers how control messages are encoded on the wire.
* Section 9 covers how data messages are encoded on the wire.
1.1. Motivation
The development of MOQT is driven by goals in a number of areas -
specifically latency, the robust feature set of QUIC and relay
support.
1.1.1. Latency
Latency is necessary to correct for variable network throughput.
Ideally live content is consumed at the same bitrate it is produced.
End-to-end latency would be fixed and only subject to encoding and
transmission delays. Unfortunately, networks have variable
throughput, primarily due to congestion. Attempting to deliver
content encoded at a higher bitrate than the network can cause
queuing along the path from producer to consumer. The speed at which
a protocol can detect and respond to congestion determines the
overall latency. TCP-based protocols are simple but are slow to
detect congestion and suffer from head-of-line blocking. Protocols
utilizing UDP directly can avoid queuing, but the application is then
responsible for the complexity of fragmentation, congestion control,
retransmissions, receiver feedback, reassembly, and more. One goal
of MOQT is to achieve the best of both these worlds: leverage the
features of QUIC to create a simple yet flexible low latency protocol
that can rapidly detect and respond to congestion.
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1.1.2. Leveraging QUIC
The parallel nature of QUIC streams can provide improvements in the
face of loss. A goal of MOQT is to design a streaming protocol to
leverage the transmission benefits afforded by parallel QUIC streams
as well exercising options for flexible loss recovery.
1.1.3. Convergence
Some live media architectures today have separate protocols for
ingest and distribution, for example RTMP and HTTP based HLS or DASH.
Switching protocols necessitates intermediary origins which re-
package the media content. While specialization can have its
benefits, there are efficiency gains to be had in not having to re-
package content. A goal of MOQT is to develop a single protocol
which can be used for transmission from contribution to distribution.
A related goal is the ability to support existing encoding and
packaging schemas, both for backwards compatibility and for
interoperability with the established content preparation ecosystem.
1.1.4. Relays
An integral feature of a protocol being successful is its ability to
deliver media at scale. Greatest scale is achieved when third-party
networks, independent of both the publisher and subscriber, can be
leveraged to relay the content. These relays must cache content for
distribution efficiency while simultaneously routing content and
deterministically responding to congestion in a multi-tenant network.
A goal of MOQT is to treat relays as first-class citizens of the
protocol and ensure that objects are structured such that information
necessary for distribution is available to relays while the media
content itself remains opaque and private.
1.2. Terms and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The following terms are used with the first letter capitalized.
Application: The entity using MoQT to transmit and receive data.
Client: The party initiating a Transport Session.
Server: The party accepting an incoming Transport Session.
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Endpoint: A Client or Server.
Peer: The other endpoint than the one being described
Publisher: An endpoint that handles subscriptions by sending
requested Objects from the requested track.
Subscriber: An endpoint that subscribes to and receives tracks.
Original Publisher: The initial publisher of a given track.
End Subscriber: A subscriber that initiates a subscription and does
not send the data on to other subscribers.
Relay: An entity that is both a Publisher and a Subscriber, but not
the Original Publisher or End Subscriber.
Upstream: In the direction of the Original Publisher
Downstream: In the direction of the End Subscriber(s)
Transport Session: A raw QUIC connection or a WebTransport session.
Congestion: Packet loss and queuing caused by degraded or overloaded
networks.
Group: A temporal sequence of objects. A group represents a join
point in a track. See (Section 2.3).
Object: An object is an addressable unit whose payload is a sequence
of bytes. Objects form the base element in the MOQT data model.
See (Section 2.1).
Track: A track is a collection of groups. See (Section 2.4).
1.3. Notational Conventions
This document uses the conventions detailed in ([RFC9000],
Section 1.3) when describing the binary encoding.
As a quick reference, the following list provides a non normative
summary of the parts of RFC9000 field syntax that are used in this
specification.
x (L): Indicates that x is L bits long
x (i): Indicates that x holds an integer value using the variable-
length encoding as described in ([RFC9000], Section 16)
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x (..): Indicates that x can be any length including zero bits long.
Values in this format always end on a byte boundary.
[x (L)]: Indicates that x is optional and has a length of L
x (L) ...: Indicates that x is repeated zero or more times and that
each instance has a length of L
This document extends the RFC9000 syntax and with the additional
field types:
x (b): Indicates that x consists of a variable length integer
encoding as described in ([RFC9000], Section 16), followed by that
many bytes of binary data
x (tuple): Indicates that x is a tuple, consisting of a variable
length integer encoded as described in ([RFC9000], Section 16),
followed by that many variable length tuple fields, each of which
are encoded as (b) above.
To reduce unnecessary use of bandwidth, variable length integers
SHOULD be encoded using the least number of bytes possible to
represent the required value.
1.3.1. Location Structure
Location identifies a particular Object in a Group within a Track.
Location {
Group (i),
Object (i)
}
Figure 1: Location structure
Location A < Location B iff
A.Group < B.Group || (A.Group == B.Group && A.Object < B.Object)
1.3.2. Key-Value-Pair Structure
Key-Value-Pair is a flexible structure designed to carry key/value
pairs in which the key is a variable length integer and the value is
either a variable length integer or a byte field of arbitrary length.
Key-Value-Pair is used in both the data plane and control plane, but
is optimized for use in the data plane.
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Key-Value-Pair {
Type (i),
[Length (i),]
Value (..)
}
Figure 2: MOQT Key-Value-Pair
* Type: an unsigned integer, encoded as a varint, identifying the
type of the value and also the subsequent serialization.
* Length: Only present when Type is odd. Specifies the length of
the Value field. The maximum length of a value is 2^16-1 bytes.
If an endpoint receives a length larger than the maximum, it MUST
close the session with a Protocol Violation.
* Value: A single varint encoded value when Type is even, otherwise
a sequence of Length bytes.
If a receiver understands a Type, and the following Value or Length/
Value does not match the serialization defined by that Type, the
receiver MUST terminate the session with error code 'Key-Value
Formatting Error'.
1.3.3. Reason Phrase Structure
Reason Phrase provides a way for the sender to encode additional
diagnostic information about the error condition, where appropriate.
Reason Phrase {
Reason Phrase Length (i),
Reason Phrase Value (..)
}
* Reason Phrase Length: A variable-length integer specifying the
length of the reason phrase in bytes. The reason phrase length
has a maximum length of 1024 bytes. If an endpoint receives a
length exceeding the maximum, it MUST close the session with a
Protocol Violation
* Reason Phrase Value: Additional diagnostic information about the
error condition. The reason phrase value is encoded as UTF-8
string and does not carry information, such as language tags, that
would aid comprehension by any entity other than the one that
created the text.
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2. Object Data Model
MOQT has a hierarchical data model, comprised of tracks which contain
groups, and groups that contain objects. Inside of a group, the
objects can be organized into subgroups.
To give an example of how an application might use this data model,
consider an application sending high and low resolution video using a
codec with temporal scalability. Each resolution is sent as a
separate track to allow the subscriber to pick the appropriate
resolution given the display environment and available bandwidth.
Each "group of pictures" in a video is sent as a group because the
first frame is needed to decode later frames. This allows the client
to join at the logical points where they can get the information to
start decoding the stream. The temporal layers are sent as separate
sub groups to allow the priority mechanism to favour the base layer
when there is not enough bandwidth to send both the base and
enhancement layers. Each frame of video on a given layer is sent as
a single object.
2.1. Objects
The basic data element of MOQT is an object. An object is an
addressable unit whose payload is a sequence of bytes. All objects
belong to a group, indicating ordering and potential dependencies.
Section 2.3 An object is uniquely identified by its track namespace,
track name, group ID, and object ID, and must be an identical
sequence of bytes regardless of how or where it is retrieved. An
Object can become unavailable, but its contents MUST NOT change over
time.
Objects are comprised of two parts: metadata and a payload. The
metadata is never encrypted and is always visible to relays (see
Section 7). The payload portion may be encrypted, in which case it
is only visible to the Original Publisher and End Subscribers. The
Original Publisher is solely responsible for the content of the
object payload. This includes the underlying encoding, compression,
any end-to-end encryption, or authentication. A relay MUST NOT
combine, split, or otherwise modify object payloads.
Objects within a Group are ordered numerically by their Object ID.
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2.2. Subgroups
A subgroup is a sequence of one or more objects from the same group
(Section 2.3) in ascending order by Object ID. Objects in a subgroup
have a dependency and priority relationship consistent with sharing a
stream and are sent on a single stream whenever possible. A Group is
delivered using at least as many streams as there are Subgroups,
typically with a one-to-one mapping between Subgroups and streams.
When a Track's forwarding preference (see Section 9.1.1) is
"Datagram", Objects are not sent in Subgroups and the description in
the remainder of this section does not apply.
Streams offer in-order reliable delivery and the ability to cancel
sending and retransmission of data. Furthermore, many
implementations offer the ability to control the relative priority of
streams, which allows control over the scheduling of sending data on
active streams.
Every object within a Group belongs to exactly one Subgroup.
Objects from two subgroups cannot be sent on the same stream.
Objects from the same Subgroup MUST NOT be sent on different streams,
unless one of the streams was reset prematurely, or upstream
conditions have forced objects from a Subgroup to be sent out of
Object ID order.
Original publishers assign each Subgroup a Subgroup ID, and do so as
they see fit. The scope of a Subgroup ID is a Group, so Subgroups
from different Groups MAY share a Subgroup ID without implying any
relationship between them. In general, publishers assign objects to
subgroups in order to leverage the features of streams as described
above.
An example strategy for using stream properties follows. If object B
is dependent on object A, then delivery of B can follow A, i.e. A and
B can be usefully delivered over a single stream. Furthermore, in
this example:
* If an object is dependent on all previous objects in a Subgroup,
it is added to that Subgroup.
* If an object is not dependent on all of the objects in a Subgroup,
it goes in a different Subgroup.
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* There are often many ways to compose Subgroups that meet these
criteria. Where possible, choose the composition that results in
the fewest Subgroups in a group to minimize the number of streams
used.
2.3. Groups
A group is a collection of objects and is a sub-unit of a track
(Section 2.4). Groups SHOULD be independently useful, so objects
within a group SHOULD NOT depend on objects in other groups. A group
provides a join point for subscriptions, so a subscriber that does
not want to receive the entire track can opt to receive only the
latest group(s). The publisher then selectively transmits objects
based on their group membership. Groups can contain any number of
objects.
2.3.1. Group Ordering
Within a track, the original publisher SHOULD publish Group IDs which
increase with time. In some cases, Groups will be produced in
increasing order, but sent to subscribers in a different order, for
example when the subscription's Group Order is Descending. Due to
network reordering and the partial reliability features of MoQT,
Groups can always be received out of order.
As a result, subscribers cannot infer the existence of a Group until
an object in the Group is received. This can create gaps in a cache
that can be filled by doing a Fetch upstream, if necessary.
Applications that cannot produce Group IDs that increase with time
are limited to the subset of MoQT that does not compare group IDs.
Subscribers to these Tracks SHOULD NOT use range filters which span
multiple Groups in FETCH or SUBSCRIBE. SUBSCRIBE and FETCH delivery
use Group Order, so a FETCH cannot deliver Groups out of order and a
subscription could have unexpected delivery order if Group IDs do not
increase with time.
2.4. Track
A track is a sequence of groups (Section 2.3). It is the entity
against which a subscriber issues a subscription request. A
subscriber can request to receive individual tracks starting at a
group boundary, including any new objects pushed by the publisher
while the track is active.
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2.4.1. Track Naming
In MOQT, every track is identified by a Full Track Name, consisting
of a Track Namespace and a Track Name.
Track Namespace is an ordered N-tuple of bytes where N can be between
1 and 32. The structured nature of Track Namespace allows relays and
applications to manipulate prefixes of a namespace. If an endpoint
receives a Track Namespace tuple with an N of 0 or more than 32, it
MUST close the session with a Protocol Violation.
Track Name is a sequence of bytes that identifies an individual track
within the namespace.
The maximum total length of a Full Track Name is 4,096 bytes,
computed as the sum of the lengths of each Track Namespace tuple
field and the Track Name length field. If an endpoint receives a
Full Track Name exceeding this length, it MUST close the session with
a Protocol Violation.
In this specification, both the Track Namespace tuple fields and the
Track Name are not constrained to a specific encoding. They carry a
sequence of bytes and comparison between two Track Namespace tuple
fields or Track Names is done by exact comparison of the bytes.
Specifications that use MoQ Transport may constrain the information
in these fields, for example by restricting them to UTF-8. Any
specification that does needs to specify the canonicalization into
the bytes in the Track Namespace or Track Name such that exact
comparison works.
2.4.2. Scope
A MOQT scope is a set of servers (as identified by their connection
URIs) for which the tuple of Track Name and Track Namespace are
guaranteed to be unique and identify a specific track. It is up to
the application using MOQT to define how broad or narrow the scope
is. An application that deals with connections between devices on a
local network may limit the scope to a single connection; by
contrast, an application that uses multiple CDNs to serve media may
require the scope to include all of those CDNs.
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Because the tuple of Track Namespace and Track Name are unique within
an MOQT scope, they can be used as a cache key for the track. If, at
a given moment in time, two tracks within the same scope contain
different data, they MUST have different names and/or namespaces.
MOQT provides subscribers with the ability to alter the specific
manner in which tracks are delivered via Subscribe Parameters, but
the actual content of the tracks does not depend on those parameters;
this is in contrast to protocols like HTTP, where request headers can
alter the server response.
3. Sessions
3.1. Session establishment
This document defines a protocol that can be used interchangeably
both over a QUIC connection directly [QUIC], and over WebTransport
[WebTransport]. Both provide streams and datagrams with similar
semantics (see [I-D.ietf-webtrans-overview], Section 4); thus, the
main difference lies in how the servers are identified and how the
connection is established. When using QUIC, datagrams MUST be
supported via the [QUIC-DATAGRAM] extension, which is already a
requirement for WebTransport over HTTP/3. The RESET_STREAM_AT
[I-D.draft-ietf-quic-reliable-stream-reset] extension to QUIC can be
used by MoQT, but the protocol is also designed to work correctly
when the extension is not supported.
There is no definition of the protocol over other transports, such as
TCP, and applications using MoQ might need to fallback to another
protocol when QUIC or WebTransport aren't available.
3.1.1. WebTransport
A MOQT server that is accessible via WebTransport can be identified
using an HTTPS URI ([RFC9110], Section 4.2.2). A MOQT session can be
established by sending an extended CONNECT request to the host and
the path indicated by the URI, as described in ([WebTransport],
Section 3).
3.1.2. QUIC
A MOQT server that is accessible via native QUIC can be identified by
a URI with a "moqt" scheme. The "moqt" URI scheme is defined as
follows, using definitions from [RFC3986]:
moqt-URI = "moqt" "://" authority path-abempty [ "?" query ]
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The authority portion MUST NOT contain an empty host portion. The
moqt URI scheme supports the /.well-known/ path prefix defined in
[RFC8615].
This protocol does not specify any semantics on the path-abempty and
query portions of the URI. The contents of those are left up to the
application.
The client can establish a connection to a MoQ server identified by a
given URI by setting up a QUIC connection to the host and port
identified by the authority section of the URI. The path-abempty and
query portions of the URI are communicated to the server using the
PATH parameter (Section 8.3.2.1) which is sent in the CLIENT_SETUP
message at the start of the session. The ALPN value [RFC7301] used
by the protocol is moq-00.
3.1.3. Connection URL
Each track MAY have one or more associated connection URLs specifying
network hosts through which a track may be accessed. The syntax of
the Connection URL and the associated connection setup procedures are
specific to the underlying transport protocol usage Section 3.
3.2. Version and Extension Negotiation
Endpoints use the exchange of Setup messages to negotiate the MOQT
version and any extensions to use.
The client indicates the MOQT versions it supports in the
CLIENT_SETUP message (see Section 8.3). It also includes the union
of all Setup Parameters Section 8.3.2 required for a handshake by any
of those versions.
Within any MOQT version, clients request the use of extensions by
adding Setup parameters corresponding to that extension. No
extensions are defined in this document.
The server replies with a SERVER_SETUP message that indicates the
chosen version, includes all parameters required for a handshake in
that version, and parameters for every extension requested by the
client that it supports.
New versions of MOQT MUST specify which existing extensions can be
used with that version. New extensions MUST specify the existing
versions with which they can be used.
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If a given parameter carries the same information in multiple
versions, but might have different optimal values in those versions,
there SHOULD be separate Setup parameters for that information in
each version.
3.3. Session initialization
The first stream opened is a client-initiated bidirectional control
stream where the endpoints exchange Setup messages (Section 8.3),
followed by other messages defined in Section 8.
This draft only specifies a single use of bidirectional streams.
Objects are sent on unidirectional streams. Because there are no
other uses of bidirectional streams, a peer MAY currently close the
session as a 'Protocol Violation' if it receives a second
bidirectional stream.
The control stream MUST NOT be closed at the underlying transport
layer while the session is active. Doing so results in the session
being closed as a 'Protocol Violation'.
3.4. Termination
The Transport Session can be terminated at any point. When native
QUIC is used, the session is closed using the CONNECTION_CLOSE frame
([QUIC], Section 19.19). When WebTransport is used, the session is
closed using the CLOSE_WEBTRANSPORT_SESSION capsule ([WebTransport],
Section 5).
The application MAY use any error message and SHOULD use a relevant
code, as defined below:
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+======+============================+
| Code | Reason |
+======+============================+
| 0x0 | No Error |
+------+----------------------------+
| 0x1 | Internal Error |
+------+----------------------------+
| 0x2 | Unauthorized |
+------+----------------------------+
| 0x3 | Protocol Violation |
+------+----------------------------+
| 0x4 | Invalid Request ID |
+------+----------------------------+
| 0x5 | Duplicate Track Alias |
+------+----------------------------+
| 0x6 | Key-Value Formatting Error |
+------+----------------------------+
| 0x7 | Too Many Requests |
+------+----------------------------+
| 0x8 | Invalid Path |
+------+----------------------------+
| 0x9 | Malformed Path |
+------+----------------------------+
| 0x10 | GOAWAY Timeout |
+------+----------------------------+
| 0x11 | Control Message Timeout |
+------+----------------------------+
| 0x12 | Data Stream Timeout |
+------+----------------------------+
| 0x13 | Auth Token Cache Overflow |
+------+----------------------------+
| 0x14 | Duplicate Auth Token Alias |
+------+----------------------------+
| 0x15 | Version Negotiation Failed |
+------+----------------------------+
Table 1
* No Error: The session is being terminated without an error.
* Internal Error: An implementation specific error occurred.
* Unauthorized: The endpoint breached an agreement, which MAY have
been pre-negotiated by the application.
* Protocol Violation: The remote endpoint performed an action that
was disallowed by the specification.
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* Invalid Request ID: The session was closed because the endpoint
used a Request ID that was smaller than or equal to a previously
received request ID, or the least-significant bit of the request
ID was incorrect for the endpoint.
* Duplicate Track Alias: The endpoint attempted to use a Track Alias
that was already in use.
* Key-Value Formatting Error: the key-value pair has a formatting
error.
* Too Many Requests: The session was closed because the endpoint
used a Request ID equal or larger than the current Maximum Request
ID.
* Invalid Path: The PATH parameter was used by a server, on a
WebTransport session, or the server does not support the path.
* Malformed Path: The PATH parameter does not conform to the rules
in Section 8.3.2.1.
* GOAWAY Timeout: The session was closed because the peer took too
long to close the session in response to a GOAWAY (Section 8.4)
message. See session migration (Section 3.5).
* Control Message Timeout: The session was closed because the peer
took too long to respond to a control message.
* Data Stream Timeout: The session was closed because the peer took
too long to send data expected on an open Data Stream Section 9.
This includes fields of a stream header or an object header within
a data stream. If an endpoint times out waiting for a new object
header on an open subgroup stream, it MAY send a STOP_SENDING on
that stream or terminate the subscription.
* Auth Token Cache Overflow - the Session limit Section 8.3.2.3 of
the size of all registered Authorization tokens has been exceeded.
* Duplicate Auth Token Alias - Authorization Token attempted to
register an alias that was in use (see Section 8.2.1.1).
* Version Negotiation Failed: The client didn't offer a version
supported by the server.
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An endpoint MAY choose to treat a subscription or request specific
error as a session error under certain circumstances, closing the
entire session in response to a condition with a single subscription
or message. Implementations need to consider the impact on other
outstanding subscriptions before making this choice.
3.5. Migration
MOQT requires a long-lived and stateful session. However, a service
provider needs the ability to shutdown/restart a server without
waiting for all sessions to drain naturally, as that can take days
for long-form media. MOQT enables proactively draining sessions via
the GOAWAY message (Section 8.4).
The server sends a GOAWAY message, signaling the client to establish
a new session and migrate any active subscriptions. The GOAWAY
message optionally contains a new URI for the new session, otherwise
the current URI is reused. The server SHOULD terminate the session
with 'GOAWAY Timeout' after a sufficient timeout if there are still
open subscriptions or fetches on a connection.
When the server is a subscriber, it SHOULD send a GOAWAY message to
downstream subscribers prior to any UNSUBSCRIBE messages to upstream
publishers.
After the client receives a GOAWAY, it's RECOMMENDED that the client
waits until there are no more active subscriptions before closing the
session with NO_ERROR. Ideally this is transparent to the
application using MOQT, which involves establishing a new session in
the background and migrating active subscriptions and announcements.
The client can choose to delay closing the session if it expects more
OBJECTs to be delivered. The server closes the session with a
'GOAWAY Timeout' if the client doesn't close the session quickly
enough.
3.6. Congestion Control
MOQT does not specify a congestion controller, but there are
important attributes to consider when selecting a congestion
controller for use with an application built on top of MOQT.
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3.6.1. Bufferbloat
Traditional AIMD congestion controllers (ex. CUBIC [RFC9438] and
Reno [RFC6582]) are prone to Bufferbloat. Bufferbloat occurs when
elements along the path build up a substantial queue of packets,
commonly more than doubling the round trip time. These queued
packets cause head-of-line blocking and latency, even when there is
no packet loss.
3.6.2. Application-Limited
The average bitrate for latency sensitive content needs to be less
than the available bandwidth, otherwise data will be queued and/or
dropped. As such, many MOQT applications will typically be limited
by the available data to send, and not the congestion controller.
Many congestion control algorithms only increase the congestion
window or bandwidth estimate if fully utilized. This combination can
lead to underestimating the available network bandwidth. As a
result, applications might need to periodically ensure the congestion
controller is not app-limited for at least a full round trip to
ensure the available bandwidth can be measured.
3.6.3. Consistent Throughput
Congestion control algorithms are commonly optimized for throughput,
not consistency. For example, BBR's PROBE_RTT state halves the
sending rate for more than a round trip in order to obtain an
accurate minimum RTT. Similarly, Reno halves it's congestion window
upon detecting loss. In both cases, the large reduction in sending
rate might cause issues with latency sensitive applications.
4. Retrieving Tracks with Subscribe and Fetch
The central interaction with a publisher is to send SUBSCRIBE and/or
FETCH for a particular track. The subscriber expects to receive a
SUBSCRIBE_OK/FETCH_OK and objects from the track.
A publisher MUST send exactly one SUBSCRIBE_OK or SUBSCRIBE_ERROR in
response to a SUBSCRIBE. It MUST send exactly one FETCH_OK or
FETCH_ERROR in response to a FETCH. The subscriber SHOULD close the
session with a protocol error if it receives more than one.
A subscriber keeps SUBSCRIBE state until it sends UNSUBSCRIBE, or
after receipt of a SUBSCRIBE_DONE or SUBSCRIBE_ERROR. Note that
SUBSCRIBE_DONE does not usually indicate that state can immediately
be destroyed, see Section 8.12.
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A subscriber keeps FETCH state until it sends FETCH_CANCEL, receives
FETCH_ERROR, or receives a FIN or RESET_STREAM for the FETCH data
stream. If the data stream is already open, it MAY send STOP_SENDING
for the data stream along with FETCH_CANCEL, but MUST send
FETCH_CANCEL.
The Publisher can destroy subscription or fetch state as soon as it
has received UNSUBSCRIBE or FETCH_CANCEL, respectively. It MUST
reset any open streams associated with the SUBSCRIBE or FETCH. It
can also destroy state after closing the FETCH data stream.
The publisher can immediately delete SUBSCRIBE state after sending
SUBSCRIBE_DONE, but MUST NOT send it until it has closed all related
streams. It can destroy all FETCH state after closing the data
stream.
A SUBSCRIBE_ERROR indicates no objects will be delivered, and both
endpoints can immediately destroy relevant state. Objects MUST NOT
be sent for requests that end with an error.
A FETCH_ERROR indicates that both endpoints can immediately destroy
state. Since a relay can start delivering FETCH objects from cache
before determining the result of the request, some objects could be
received even if the FETCH results in error.
The Parameters in SUBSCRIBE and FETCH MUST NOT cause the publisher to
alter the payload of the objects it sends, as that would violate the
track uniqueness guarantee described in Section 2.4.2.
5. Namespace Discovery
Discovery of MoQT servers is always done out-of-band. Namespace
discovery can be done in the context of an established MoQT session.
Given sufficient out of band information, it is valid for a
subscriber to send a SUBSCRIBE or FETCH message to a publisher
(including a relay) without any previous MoQT messages besides SETUP.
However, SUBSCRIBE_ANNOUNCES and ANNOUNCE messages provide an in-band
means of discovery of publishers for a namespace.
The syntax of these messages is described in Section 8.
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5.1. Subscribing to Announcements
If the subscriber is aware of a namespace of interest, it can send
SUBSCRIBE_ANNOUNCES to publishers/relays it has established a session
with. The recipient of this message will send any relevant ANNOUNCE
or UNANNOUNCE messages for that namespace, or more specific part of
that namespace.
A publisher MUST send exactly one SUBSCRIBE_ANNOUNCES_OK or
SUBSCRIBE_ANNOUNCES_ERROR in response to a SUBSCRIBE_ANNOUNCES. The
subscriber SHOULD close the session with a protocol error if it
detects receiving more than one.
The receiver of a SUBSCRIBE_ANNOUNCES_OK or SUBSCRIBE_ANNOUNCES_ERROR
ought to forward the result to the application, so the application
can decide which other publishers to contact, if any.
An UNSUBSCRIBE_ANNOUNCES withdraws a previous SUBSCRIBE_ANNOUNCES.
It does not prohibit the receiver (publisher) from sending further
ANNOUNCE messages.
5.2. Announcements
A publisher MAY send ANNOUNCE messages to any subscriber. An
ANNOUNCE indicates to the subscriber that the publisher has tracks
available in that namespace. A subscriber MAY send SUBSCRIBE or
FETCH for a namespace without having received an ANNOUNCE for it.
If a publisher is authoritative for a given namespace, or is a relay
that has received an authorized ANNOUNCE for that namespace from an
upstream publisher, it MUST send an ANNOUNCE to any subscriber that
has subscribed to ANNOUNCE for that namespace, or a more generic set
including that namespace. A publisher MAY send the ANNOUNCE to any
other subscriber.
An endpoint SHOULD NOT, however, send an ANNOUNCE advertising a
namespace that exactly matches a namespace for which the peer sent an
earlier ANNOUNCE (i.e. an ANNOUNCE ought not to be echoed back to its
sender).
The receiver of an ANNOUNCE_OK or ANNOUNCE_ERROR SHOULD report this
to the application to inform the search for additional subscribers
for a namespace, or abandoning the attempt to publish under this
namespace. This might be especially useful in upload or chat
applications. A subscriber MUST send exactly one ANNOUNCE_OK or
ANNOUNCE_ERROR in response to an ANNOUNCE. The publisher SHOULD
close the session with a protocol error if it receives more than one.
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An UNANNOUNCE message withdraws a previous ANNOUNCE, although it is
not a protocol error for the subscriber to send a SUBSCRIBE or FETCH
message after receiving an UNANNOUNCE.
A subscriber can send ANNOUNCE_CANCEL to revoke acceptance of an
ANNOUNCE, for example due to expiration of authorization credentials.
The message enables the publisher to ANNOUNCE again with refreshed
authorization, or discard associated state. After receiving an
ANNOUNCE_CANCEL, the publisher does not send UNANNOUNCE.
While ANNOUNCE indicates to relays how to connect publishers and
subscribers, it is not a full-fledged routing protocol and does not
protect against loops and other phenomena. In particular, ANNOUNCE
SHOULD NOT be used to find paths through richly connected networks of
relays.
A subscriber MAY send a SUBSCRIBE or FETCH for a track to any
publisher. If it has accepted an ANNOUNCE with a namespace that
exactly matches the namespace for that track, it SHOULD only request
it from the senders of those ANNOUNCE messages.
6. Priorities
MoQ priorities allow a subscriber and original publisher to influence
the transmission order of Objects within a session in the presence of
congestion.
6.1. Definitions
MoQT maintains priorities between different _schedulable objects_. A
schedulable object in MoQT is either:
1. An object in response to a SUBSCRIBE that belongs to a subgroup
where that object is the next object in that subgroup.
2. An object in response to a SUBSCRIBE that belongs to a track with
delivery preference Datagram.
3. An object in response to a FETCH where that object is the next
object in the response.
A single subgroup or datagram has a single publisher priority.
Within a response to SUBSCRIBE, it can be useful to conceptualize
this process as scheduling subgroups or datagrams instead of
individual objects on them. FETCH responses however can contain
objects with different publisher priorities.
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A _priority number_ is an unsigned integer with a value between 0 and
255. A lower priority number indicates higher priority; the highest
priority is 0.
_Subscriber Priority_ is a priority number associated with an
individual request. It is specified in the SUBSCRIBE or FETCH
message, and can be updated via SUBSCRIBE_UPDATE message. The
subscriber priority of an individual schedulable object is the
subscriber priority of the request that caused that object to be
sent. When subscriber priority is changed, a best effort SHOULD be
made to apply the change to all objects that have not been sent, but
it is implementation dependent what happens to objects that have
already been received and possibly scheduled.
_Publisher Priority_ is a priority number associated with an
individual schedulable object. It is specified in the header of the
respective subgroup or datagram, or in each object in a FETCH
response.
_Group Order_ is a property of an individual subscription. It can be
either 'Ascending' (groups with lower group ID are sent first), or
'Descending' (groups with higher group ID are sent first). The
subscriber optionally communicates its group order preference in the
SUBSCRIBE message; the publisher's preference is used if the
subscriber did not express one (by setting Group Order field to value
0x0). The group order of an existing subscription cannot be changed.
6.2. Scheduling Algorithm
When an MoQT publisher has multiple schedulable objects it can choose
between, the objects SHOULD be selected as follows:
1. If two objects have a different subscriber priority associated
with them, the one with *the highest subscriber priority* is sent
first.
2. If two objects have the same subscriber priority, but a different
publisher priority, the one with *the highest publisher priority*
is sent first.
3. If two objects in response to the same request have the same
subscriber and publisher priority, but belong to two different
groups of the same track, *the group order* of the associated
subscription is used to decide the one that is sent first.
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4. If two objects in response to the same request belong to the same
group of the same track, the one with *the lowest Subgroup ID*
(for tracks with delivery preference Subgroup), or *the lowest
Object ID* (for tracks with delivery preference Datagram) is sent
first.
The definition of "sent first" in the algorithm is implementation
dependent and is constrained by the prioritization interface of the
underlying transport. For some implementations, it could mean that
the object is serialized and passed to the underlying transport
first. In other implementations, it can control the order packets
are initially transmitted.
This algorithm does not provide a well-defined ordering for objects
that belong to different subscriptions or FETCH responses, but have
the same subscriber and publisher priority. The ordering in those
cases is implementation-defined, though the expectation is that all
subscriptions will be able to send some data.
Given the critical nature of control messages and their relatively
small size, the control stream SHOULD be prioritized higher than all
subscribed Objects.
6.3. Considerations for Setting Priorities
Relays SHOULD respect the subscriber and original publisher's
priorities. Relays can receive subscriptions with conflicting
subscriber priorities or Group Order preferences. Relays SHOULD NOT
directly use Subscriber Priority or Group Order from incoming
subscriptions for upstream subscriptions. Relays use of these fields
for upstream subscriptions can be based on factors specific to it,
such as the popularity of the content or policy, or relays can
specify the same value for all upstream subscriptions.
MoQ Sessions can span multiple namespaces, and priorities might not
be coordinated across namespaces. The subscriber's priority is
considered first, so there is a mechanism for a subscriber to fix
incompatibilities between different namespaces prioritization
schemes. Additionally, it is anticipated that when multiple
namespaces are present within a session, the namespaces could be
coordinating, possibly part of the same application. In cases when
pooling among namespaces is expected to cause issues, multiple MoQ
sessions, either within a single connection or on multiple
connections can be used.
Implementations that have a default priority SHOULD set it to a value
in the middle of the range (eg: 128) to allow non-default priorities
to be set either higher or lower.
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7. Relays
Relays are leveraged to enable distribution scale in the MoQ
architecture. Relays can be used to form an overlay delivery
network, similar in functionality to Content Delivery Networks
(CDNs). Additionally, relays serve as policy enforcement points by
validating subscribe and publish requests at the edge of a network.
Relays are endpoints, which means they terminate Transport Sessions
in order to have visibility of MoQ Object metadata.
7.1. Caching Relays
Relays MAY cache Objects, but are not required to.
A caching relay saves Objects to its cache identified by the Object's
Full Track Name, Group ID and Object ID. If multiple objects are
received with the same Full Track Name, Group ID and Object ID,
Relays MAY ignore subsequently received Objects or MAY use them to
update certain cached fields. Implementations that update the cache
need to protect against cache poisoning. The only Object fields that
can be updated are the following:
1. Object Status can transition from any status to Object Does Not
Exist in cases where the object is no longer available.
Transitions between Normal, End of Group and End of Track are
invalid.
2. Object Header Extensions can be added, removed or updated,
subject to the constraints of the specific header extension.
An endpoint that receives a duplicate Object with an invalid Object
Status change, or a Forwarding Preference, Subgroup ID, Priority or
Payload that differ from a previous version MUST treat the track as
Malformed.
Note that due to reordering, an implementation can receive a
duplicate Object with a status of Normal, End of Group or End of
Track after receiving a previous status of Object Not Exists. The
endpoint SHOULD NOT cache or forward the duplicate object in this
case.
A cache MUST store all properties of an Object defined in
Section 9.1.1, with the exception of any extensions (Section 9.1.1.2)
that specify otherwise.
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7.2. Subscriber Interactions
Subscribers subscribe to tracks by sending a SUBSCRIBE (Section 8.7)
control message for each track of interest. Relays MUST ensure
subscribers are authorized to access the content associated with the
track. The authorization information can be part of subscription
request itself or part of the encompassing session. The specifics of
how a relay authorizes a user are outside the scope of this
specification.
The relay will have to send an upstream SUBSCRIBE and/or FETCH if it
does not have all the objects in the FETCH, or is not currently
subscribed to the full requested range. In this case, it SHOULD
withhold sending its own SUBSCRIBE_OK until receiving one from
upstream. It MUST withhold FETCH_OK until receiving one from
upstream.
For successful subscriptions, the publisher maintains a list of
subscribers for each track. Each new Object belonging to the track
within the subscription range is forwarded to each active subscriber,
dependent on the congestion response.
Relays MUST be able to process objects for the same Full Track Name
from multiple publishers and forward objects to active matching
subscriptions. The same object SHOULD NOT be forwarded more than
once on the same subscription.
A relay MUST NOT reorder or drop objects received on a multi-object
stream when forwarding to subscribers, unless it has application
specific information.
Relays MAY aggregate authorized subscriptions for a given track when
multiple subscribers request the same track. Subscription
aggregation allows relays to make only a single upstream subscription
for the track. The published content received from the upstream
subscription request is cached and shared among the pending
subscribers. Because SUBSCRIBE_UPDATE only allows narrowing a
subscription, relays that aggregate upstream subscriptions can
subscribe using the Latest Object filter to avoid churn as downstream
subscribers with disparate filters subscribe and unsubscribe from a
track.
7.2.1. Graceful Subscriber Relay Switchover
This section describes behavior a subscriber MAY implement to allow
for a better user experience when a relay sends a GOAWAY.
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When a subscriber receives the GOAWAY message, it starts the process
of connecting to a new relay and sending the SUBSCRIBE requests for
all active subscriptions to the new relay. The new relay will send a
response to the subscribes and if they are successful, the
subscriptions to the old relay can be stopped with an UNSUBSCRIBE.
7.3. Publisher Interactions
Publishing through the relay starts with publisher sending ANNOUNCE
control message with a Track Namespace (Section 2.4). The ANNOUNCE
enables the relay to know which publisher to forward a SUBSCRIBE to.
Relays MUST verify that publishers are authorized to publish the
content associated with the set of tracks whose Track Namespace
matches the announced namespace. Where the authorization and
identification of the publisher occurs depends on the way the relay
is managed and is application specific.
A Relay can receive announcements from multiple publishers for the
same Track Namespace and it SHOULD respond with the same response to
each of the publishers, as though it was responding to an ANNOUNCE
from a single publisher for a given track namespace.
When a publisher wants to stop new subscriptions for an announced
namespace it sends an UNANNOUNCE. A subscriber indicates it will no
longer subcribe to tracks in a namespace it previously responded
ANNOUNCE_OK to by sending an ANNOUNCE_CANCEL.
A relay manages sessions from multiple publishers and subscribers,
connecting them based on the track namespace. This MUST use an exact
match on track namespace unless otherwise negotiated by the
application. For example, a SUBSCRIBE namespace=foobar message will
be forwarded to the session that sent ANNOUNCE namespace=foobar.
When a relay receives an incoming SUBSCRIBE request that triggers an
upstream subscription, it SHOULD send a SUBSCRIBE request to each
publisher that has announced the subscription's namespace, unless it
already has an active subscription for the Objects requested by the
incoming SUBSCRIBE request from all available publishers.
When a relay receives an incoming ANNOUNCE for a given namespace, for
each active upstream subscription that matches that namespace, it
SHOULD send a SUBSCRIBE to the publisher that sent the ANNOUNCE.
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Object headers carry a short hop-by-hop Track Alias that maps to the
Full Track Name (see Section 8.8). Relays use the Track Alias of an
incoming Object to identify its track and find the active subscribers
for that track. Relays MUST forward Objects to matching subscribers
in accordance to each subscription's priority, group order, and
delivery timeout.
If an upstream session is closed due to an unknown or invalid control
message or Object, the relay MUST NOT continue to propagate that
message or Object downstream, because it would enable a single
session to close unrelated sessions.
7.3.1. Graceful Publisher Network Switchover
This section describes behavior that a publisher MAY choose to
implement to allow for a better users experience when switching
between networks, such as WiFi to Cellular or vice versa.
If the original publisher detects it is likely to need to switch
networks, for example because the WiFi signal is getting weaker, and
it does not have QUIC connection migration available, it establishes
a new session over the new interface and sends an ANNOUNCE. The
relay will forward matching subscribes and the publisher publishes
objects on both sessions. Once the subscriptions have migrated over
to session on the new network, the publisher can stop publishing
objects on the old network. The relay will drop duplicate objects
received on both subscriptions. Ideally, the subscriptions
downstream from the relay do no observe this change, and keep
receiving the objects on the same subscription.
7.3.2. Graceful Publisher Relay Switchover
This section describes behavior that a publisher MAY choose to
implement to allow for a better user experience when a relay sends
them a GOAWAY.
When a publisher receives a GOAWAY, it starts the process of
connecting to a new relay and sends announces, but it does not
immediately stop publishing objects to the old relay. The new relay
will send subscribes and the publisher can start sending new objects
to the new relay instead of the old relay. Once objects are going to
the new relay, the announcement and subscription to the old relay can
be stopped.
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7.4. Relay Object Handling
MOQT encodes the delivery information via Object headers
(Section 9.1). A relay MUST NOT modify Object properties when
forwarding.
A relay MUST treat the object payload as opaque. A relay MUST NOT
combine, split, or otherwise modify object payloads. A relay SHOULD
prioritize sending Objects based on Section 6.
A publisher SHOULD begin sending incomplete objects when available to
avoid incurring additional latency.
A relay that reads from one stream and writes to another in order can
introduce head-of-line blocking. Packet loss will cause stream data
to be buffered in the library, awaiting in-order delivery, which
could increase latency over additional hops. To mitigate this, a
relay MAY read and write stream data out of order subject to flow
control limits. See section 2.2 in [QUIC].
8. Control Messages
MOQT uses a single bidirectional stream to exchange control messages,
as defined in Section 3.3. Every single message on the control
stream is formatted as follows:
MOQT Control Message {
Message Type (i),
Message Length (16),
Message Payload (..),
}
Figure 3: MOQT Message
The following Message Types are defined:
+======+============================================+
| ID | Messages |
+======+============================================+
| 0x01 | RESERVED (SETUP for version 00) |
+------+--------------------------------------------+
| 0x40 | RESERVED (CLIENT_SETUP for versions <= 10) |
+------+--------------------------------------------+
| 0x41 | RESERVED (SERVER_SETUP for versions <= 10) |
+------+--------------------------------------------+
| 0x20 | CLIENT_SETUP (Section 8.3) |
+------+--------------------------------------------+
| 0x21 | SERVER_SETUP (Section 8.3) |
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+------+--------------------------------------------+
| 0x10 | GOAWAY (Section 8.4) |
+------+--------------------------------------------+
| 0x15 | MAX_REQUEST_ID (Section 8.5) |
+------+--------------------------------------------+
| 0x1A | REQUESTS_BLOCKED (Section 8.6) |
+------+--------------------------------------------+
| 0x3 | SUBSCRIBE (Section 8.7) |
+------+--------------------------------------------+
| 0x4 | SUBSCRIBE_OK (Section 8.8) |
+------+--------------------------------------------+
| 0x5 | SUBSCRIBE_ERROR (Section 8.9) |
+------+--------------------------------------------+
| 0xA | UNSUBSCRIBE (Section 8.11) |
+------+--------------------------------------------+
| 0x2 | SUBSCRIBE_UPDATE (Section 8.10) |
+------+--------------------------------------------+
| 0xB | SUBSCRIBE_DONE (Section 8.12) |
+------+--------------------------------------------+
| 0x16 | FETCH (Section 8.13) |
+------+--------------------------------------------+
| 0x18 | FETCH_OK (Section 8.14) |
+------+--------------------------------------------+
| 0x19 | FETCH_ERROR (Section 8.15) |
+------+--------------------------------------------+
| 0x17 | FETCH_CANCEL (Section 8.16) |
+------+--------------------------------------------+
| 0xD | TRACK_STATUS_REQUEST (Section 8.17) |
+------+--------------------------------------------+
| 0xE | TRACK_STATUS (Section 8.18) |
+------+--------------------------------------------+
| 0x6 | ANNOUNCE (Section 8.19) |
+------+--------------------------------------------+
| 0x7 | ANNOUNCE_OK (Section 8.20) |
+------+--------------------------------------------+
| 0x8 | ANNOUNCE_ERROR (Section 8.21) |
+------+--------------------------------------------+
| 0x9 | UNANNOUNCE (Section 8.22) |
+------+--------------------------------------------+
| 0xC | ANNOUNCE_CANCEL (Section 8.23) |
+------+--------------------------------------------+
| 0x11 | SUBSCRIBE_ANNOUNCES (Section 8.24) |
+------+--------------------------------------------+
| 0x12 | SUBSCRIBE_ANNOUNCES_OK (Section 8.25) |
+------+--------------------------------------------+
| 0x13 | SUBSCRIBE_ANNOUNCES_ERROR (Section 8.26 |
+------+--------------------------------------------+
| 0x14 | UNSUBSCRIBE_ANNOUNCES (Section 8.27) |
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+------+--------------------------------------------+
Table 2
An endpoint that receives an unknown message type MUST close the
session. Control messages have a length to make parsing easier, but
no control messages are intended to be ignored. The length is set to
the number of bytes in Message Payload, which is defined by each
message type. If the length does not match the length of the Message
Payload, the receiver MUST close the session with Protocol Violation.
8.1. Request ID
Most MoQT control messages contain a session specific Request ID.
The Request ID correlates requests and responses, allows endpoints to
update or terminate ongoing requests, and supports the endpoint's
ability to limit the concurrency and frequency of requests. There
are independent Request IDs for each endpoint. The client's Request
ID starts at 0 and are even and the server's Request ID starts at 1
and are odd. The Request ID increments by 2 with ANNOUNCE, FETCH,
SUBSCRIBE, SUBSCRIBE_ANNOUNCES or TRACK_STATUS request. If an
endpoint receives a Request ID that is not valid for the peer, or a
new request with a Request ID that is not expected, it MUST close the
session with Invalid Request ID.
8.2. Parameters
Some messages include a Parameters field that encode optional message
elements.
Senders MUST NOT repeat the same parameter type in a message unless
the parameter definition explicitly allows multiple instances of that
type to be sent in a single message. Receivers SHOULD check that
there are no unauthorized duplicate parameters and close the session
as a 'Protocol Violation' if found. Receivers MUST allow duplicates
of unknown parameters.
Receivers ignore unrecognized parameters.
The number of parameters in a message is not specifically limited,
but the total length of a control message is limited to 2^16-1.
Parameters are serialized as Key-Value-Pairs Figure 2.
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Setup message parameters use a namespace that is constant across all
MoQ Transport versions. All other messages use a version-specific
namespace. For example, the integer '1' can refer to different
parameters for Setup messages and for all other message types. SETUP
message parameter types are defined in Section 8.3.2. Version-
specific parameter types are defined in Section 8.2.1.
8.2.1. Version Specific Parameters
Each version-specific parameter definition indicates the message
types in which it can appear. If it appears in some other type of
message, it MUST be ignored. Note that since Setup parameters use a
separate namespace, it is impossible for these parameters to appear
in Setup messages.
8.2.1.1. AUTHORIZATION TOKEN
The AUTHORIZATION TOKEN parameter (Parameter Type 0x01) identifies a
track's authorization information in a SUBSCRIBE,
SUBSCRIBE_ANNOUNCES, ANNOUNCE TRACK_STATUS_REQUEST or FETCH message.
This parameter is populated for cases where the authorization is
required at the track or namespace level.
The AUTHORIZATION TOKEN parameter MAY be repeated within a message.
The TOKEN value is a structured object containing an optional
session-specific alias. The Alias allows the client to reference a
previously transmitted TOKEN in future messages. The TOKEN value is
serialized as follows:
TOKEN {
Alias Type (i),
[Token Alias (i),]
[Token Type (i),]
[Token Value (..)]
}
Figure 4: AUTHORIZATION TOKEN value
* Alias Type - an integer defining both the serialization and the
processing behavior of the receiver. This Alias type has the
following code points:
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+======+===========+===============================+
| Code | Name | Serialization and behavior |
+======+===========+===============================+
| 0x0 | DELETE | There is an Alias but no Type |
| | | or Value. This Alias |
+------+-----------+-------------------------------+
| | | and the Token Value it was |
| | | previously associated with |
+------+-----------+-------------------------------+
| | | MUST be retired. Retiring |
| | | removes them from the pool |
+------+-----------+-------------------------------+
| | | of actively registered |
| | | tokens. |
+------+-----------+-------------------------------+
| 0x1 | REGISTER | There is an Alias, a Type and |
| | | a Value. This Alias |
+------+-----------+-------------------------------+
| | | MUST be associated with the |
| | | Token Value for the |
+------+-----------+-------------------------------+
| | | duration of the Session or it |
| | | is deleted. This action |
+------+-----------+-------------------------------+
| | | is termed "registering" the |
| | | Token. |
+------+-----------+-------------------------------+
| 0x2 | USE_ALIAS | There is an Alias but no Type |
| | | or Value. Use the Token |
+------+-----------+-------------------------------+
| | | Type and Value previously |
| | | registered with this Alias. |
+------+-----------+-------------------------------+
| 0x3 | USE_VALUE | There is no Alias and there |
| | | is a Type and Value. Use |
+------+-----------+-------------------------------+
| | | the Token Value as provided. |
| | | The Token Value may be |
+------+-----------+-------------------------------+
| | | discarded after processing. |
+------+-----------+-------------------------------+
Table 3
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* Token Alias - a session-specific integer identifier that
references a Token Value. The Token Alias MUST be unique within
the Session. Once a Token Alias has been registered, it cannot be
re-registered within the Session without first being deleted. Use
of the Token Alias is optional.
* Token Type - a numeric identifier for the type of Token payload
being transmitted. This type is defined by the IANA table "MOQT
Auth Token Type". See Section 12. Type 0 is reserved to indicate
that the type is not defined in the table and must be negotiated
out-of-band between client and receiver.
* Token Value - the payload of the Token. The contents and
serialization of this payload are defined by the Token Type.
The receiver of a message containing an invalid AUTHORIZATION TOKEN
parameter MUST reject that message with an Malformed Auth Token
error. This can be due to invalid serialization or providing a token
value which does not match the declared Token Type. The receiver of
a message referencing an alias that is not currently registered MUST
reject the message with Unknown Auth Token Alias. The receiver of a
message attempting to register an alias which is already registered
MUST close the session with Duplicate Auth Token Alias.
Any message carrying an AUTHORIZATION TOKEN with Alias Type REGISTER
that does not result in Malformed Auth Token MUST effect the token
registration, even if the message fails for other reasons, including
Unauthorized. This allows senders to pipeline messages that refer to
previously registered tokens.
If a receiver detects that an authorization token has expired, it
MUST retain the registered alias until it is deleted by the sender,
though it MAY discard other state associated with the token that is
no longer needed. Expiration does not affect the size occupied by a
token in the token cache. Any message that references the token with
Alias Type USE_ALIAS fails with Expired Auth Token.
Using an Alias to refer to a previously registered Token Value is for
efficiency only and has the same effect as if the Token Value was
included directly. Retiring an Alias that was previously used to
authorize a message has no retroactive effect on the original
authorization, nor does it prevent that same Token Value being re-
registered.
Clients SHOULD only register tokens which they intend to re-use
during the session. Client SHOULD retire previously registered
tokens once their utility has passed.
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By registering a Token, the client is requiring the receiver to store
the Token Alias and Token Value until they are retired, or the
Session ends. The receiver can protect its resources by sending a
SETUP parameter defining the MAX_AUTH_TOKEN_CACHE_SIZE
Section 8.3.2.3 limit it is willing to accept. If a registration is
attempted which would cause this limit to be exceeded, the receiver
MUST termiate the Session with a Auth Token Cache Overflow error.
8.2.1.2. DELIVERY TIMEOUT Parameter
The DELIVERY TIMEOUT parameter (Parameter Type 0x02) MAY appear in a
TRACK_STATUS, SUBSCRIBE, SUBSCRIBE_OK, or a SUBSCRIBE_UDPATE message.
It is the duration in milliseconds the relay SHOULD continue to
attempt forwarding Objects after they have been received. The start
time for the timeout is based on when the beginning of the Object is
received, and does not depend upon the forwarding preference. There
is no explicit signal that an Object was not sent because the
delivery timeout was exceeded.
If both the subscriber and publisher specify the parameter, they use
the min of the two values for the subscription. The publisher SHOULD
always specify the value received from an upstream subscription when
there is one, and nothing otherwise. If an earlier Object arrives
later than subsequent Objects, relays can consider the receipt time
as that of the next later Object, with the assumption that the
Object's data was reordered.
If neither the subscriber or publisher specify DELIVERY TIMEOUT, all
Objects in the track matching the subscription filter are delivered
as indicated by their Group Order and Priority. If a subscriber
exceeds the publisher's resource limits by failing to consume objects
at a sufficient rate, the publisher MAY terminate the subscription
with error 'Too Far Behind'.
If an object in a subgroup exceeds the delivery timeout, the
publisher MUST reset the underlying transport stream (see
Section 9.4.3).
When sent by a subscriber, this parameter is intended to be specific
to a subscription, so it SHOULD NOT be forwarded upstream by a relay
that intends to serve multiple subscriptions for the same track.
Publishers SHOULD consider whether the entire Object is likely to be
delivered before sending any data for that Object, taking into
account priorities, congestion control, and any other relevant
information.
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8.2.1.3. MAX CACHE DURATION Parameter
The MAX_CACHE_DURATION parameter (Parameter Type 0x04) MAY appear in
a SUBSCRIBE_OK, FETCH_OK or TRACK_STATUS message. It is an integer
expressing the number of milliseconds an object can be served from a
cache. If present, the relay MUST NOT start forwarding any
individual Object received through this subscription or fetch after
the specified number of milliseconds has elapsed since the beginning
of the Object was received. This means Objects earlier in a multi-
object stream will expire earlier than Objects later in the stream.
Once Objects have expired from cache, their state becomes unknown,
and a relay that handles a downstream request that includes those
Objects re-requests them.
8.3. CLIENT_SETUP and SERVER_SETUP
The CLIENT_SETUP and SERVER_SETUP messages are the first messages
exchanged by the client and the server; they allow the endpoints to
establish the mutually supported version and agree on the initial
configuration before any objects are exchanged. It is a sequence of
key-value pairs called Setup parameters; the semantics and format of
which can vary based on whether the client or server is sending. To
ensure future extensibility of MOQT, endpoints MUST ignore unknown
setup parameters. TODO: describe GREASE for those.
The wire format of the Setup messages are as follows:
CLIENT_SETUP Message {
Type (i) = 0x20,
Length (i),
Number of Supported Versions (i),
Supported Versions (i) ...,
Number of Parameters (i),
Setup Parameters (..) ...,
}
SERVER_SETUP Message {
Type (i) = 0x21,
Length (i),
Selected Version (i),
Number of Parameters (i),
Setup Parameters (..) ...,
}
Figure 5: MOQT Setup Messages
The available versions and Setup parameters are detailed in the next
sections.
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8.3.1. Versions
MoQ Transport versions are a 32-bit unsigned integer, encoded as a
varint. This version of the specification is identified by the
number 0x00000001. Versions with the most significant 16 bits of the
version number cleared are reserved for use in future IETF consensus
documents.
The client offers the list of the protocol versions it supports; the
server MUST reply with one of the versions offered by the client. If
the server does not support any of the versions offered by the
client, or the client receives a server version that it did not
offer, the corresponding peer MUST close the session with Version
Negotiation Failed.
[[RFC editor: please remove the remainder of this section before
publication.]]
The version number for the final version of this specification
(0x00000001), is reserved for the version of the protocol that is
published as an RFC. Version numbers used to identify IETF drafts
are created by adding the draft number to 0xff000000. For example,
draft-ietf-moq-transport-13 would be identified as 0xff00000D.
8.3.2. Setup Parameters
8.3.2.1. PATH
The PATH parameter (Parameter Type 0x01) allows the client to specify
the path of the MoQ URI when using native QUIC ([QUIC]). It MUST NOT
be used by the server, or when WebTransport is used. If the peer
receives a PATH parameter from the server, when WebTransport is used,
or one the server does not support, it MUST close the session with
Invalid Path.
The PATH parameter follows the URI formatting rules [RFC3986]. When
connecting to a server using a URI with the "moqt" scheme, the client
MUST set the PATH parameter to the path-abempty portion of the URI;
if query is present, the client MUST concatenate ?, followed by the
query portion of the URI to the parameter. If a PATH does not
conform to these rules, the session MUST be closed with Malformed
Path.
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8.3.2.2. MAX_REQUEST_ID
The MAX_REQUEST_ID parameter (Parameter Type 0x02) communicates an
initial value for the Maximum Request ID to the receiving endpoint.
The default value is 0, so if not specified, the peer MUST NOT send
requests.
8.3.2.3. MAX_AUTH_TOKEN_CACHE_SIZE
The MAX_AUTH_TOKEN_CACHE_SIZE parameter (Parameter Type 0x04)
communicates the maximum size in bytes of all actively registered
Authorization tokens that the server is willing to store per Session.
This parameter is optional. The default value is 0 which prohibits
the use of token aliases.
The token size is calculated as 8 bytes + the size of the Token value
(see Figure 4). The total size as restricted by the
MAX_AUTH_TOKEN_CACHE_SIZE parameter is calculated as the sum of the
token sizes for all registered tokens (Alias Type value of 0x01)
minus the sum of the token sizes for all deregistered tokens (Alias
Type value of 0x00), since Session initiation.
8.4. GOAWAY
An endpoint sends a GOAWAY message to inform the peer it intends to
close the session soon. Servers can use GOAWAY to initiate session
migration (Section 3.5) with an optional URI.
The GOAWAY message does not impact subscription state. A subscriber
SHOULD individually UNSUBSCRIBE for each existing subscription, while
a publisher MAY reject new requests while in the draining state.
Upon receiving a GOAWAY, an endpoint SHOULD NOT initiate new requests
to the peer including SUBSCRIBE, FETCH, ANNOUNCE and
SUBSCRIBE_ANNOUNCE.
The endpoint MUST terminate the session with a Protocol Violation
(Section 3.4) if it receives multiple GOAWAY messages.
GOAWAY Message {
Type (i) = 0x10,
Length (i),
New Session URI Length (i),
New Session URI (..),
}
Figure 6: MOQT GOAWAY Message
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* New Session URI: When received by a client, indicates where the
client can connect to continue this session. The client MUST use
this URI for the new session if provided. If the URI is zero
bytes long, the current URI is reused instead. The new session
URI SHOULD use the same scheme as the current URL to ensure
compatibility. The maxmimum length of the New Session URI is
8,192 bytes. If an endpoint receives a length exceeding the
maximum, it MUST close the session with a Protocol Violation.
If a server receives a GOAWAY with a non-zero New Session URI
Length it MUST terminate the session with a Protocol Violation.
8.5. MAX_REQUEST_ID
An endpoint sends a MAX_REQUEST_ID message to increase the number of
requests the peer can send within a session.
The Maximum Request ID MUST only increase within a session, and
receipt of a MAX_REQUEST_ID message with an equal or smaller Request
ID value is a 'Protocol Violation'.
MAX_REQUEST_ID Message {
Type (i) = 0x15,
Length (i),
Request ID (i),
}
Figure 7: MOQT MAX_REQUEST_ID Message
* Request ID: The new Maximum Request ID for the session. If a
Request ID equal or larger than this is received by the endpoint
that sent the MAX_REQUEST_ID in any request message (ANNOUNCE,
FETCH, SUBSCRIBE, SUBSCRIBE_ANNOUNCES or TRACK_STATUS_REQUEST),
the endpoint MUST close the session with an error of 'Too Many
Requests'.
MAX_REQUEST_ID is similar to MAX_STREAMS in ([RFC9000], Section 4.6),
and similar considerations apply when deciding how often to send
MAX_REQUEST_ID. For example, implementations might choose to
increase MAX_REQUEST_ID as subscriptions close to keep the number of
subscriptions available roughly consistent.
8.6. REQUESTS_BLOCKED
The REQUESTS_BLOCKED message is sent when an endpoint would like to
send a new request, but cannot because the Request ID would exceed
the Maximum Request ID value sent by the peer. The endpoint SHOULD
send only one REQUESTS_BLOCKED for a given Maximum Request ID.
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An endpoint MAY send a MAX_REQUEST_ID upon receipt of
REQUESTS_BLOCKED, but it MUST NOT rely on REQUESTS_BLOCKED to trigger
sending a MAX_REQUEST_ID, because sending REQUESTS_BLOCKED is not
required.
REQUESTS_BLOCKED Message {
Type (i) = 0x1A,
Length (i),
Maximum Request ID (i),
}
Figure 8: MOQT REQUESTS_BLOCKED Message
* Maximum Request ID: The Maximum Request ID for the session on
which the endpoint is blocked. More on Request ID in Section 8.7.
8.7. SUBSCRIBE
A subscription causes the publisher to send newly published objects
for a track. A subscriber MUST NOT make multiple active
subscriptions for a track within a single session and publishers
SHOULD treat this as a protocol violation.
*Filter Types*
The subscriber specifies a filter on the subscription to allow the
publisher to identify which objects need to be delivered.
All filters have a Start Location and an optional End Group. Only
objects published or received via a subscription having Locations
greater than or equal to Start and strictly less than or equal to the
End Group (when present) pass the filter.
The Largest Object is defined to be the object with the largest
Location (Section 1.3.1) in the track from the perspective of the
endpoint processing the SUBSCRIBE message.
There are 4 types of filters:
Latest Object (0x2): The filter Start Location is {Largest
Object.Group, Largest Object.Object + 1} and Largest Object is
communicated in SUBSCRIBE_OK. If no content has been delivered yet,
the filter Start Location is {0, 0}. There is no End Group - the
subscription is open ended. Note that due to network reordering or
prioritization, relays can receive Objects with Locations smaller
than Largest Object after the SUBSCRIBE is processed, but these
Objects do not pass the Latest Object filter.
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Next Group Start (0x1): The filter start Location is {Largest
Object.Group + 1, 0} and Largest Object is communicated in
SUBSCRIBE_OK. If no content has been delivered yet, the filter Start
Location is {0, 0}. There is no End Group - the subscription is open
ended. For scenarios where the subscriber intends to start more than
one group in the future, it can use an AbsoluteStart filter instead.
AbsoluteStart (0x3): The filter Start Location is specified
explicitly in the SUBSCRIBE message. The Start specified in the
SUBSCRIBE message MAY be less than the Largest Object observed at the
publisher. There is no End Group - the subscription is open ended.
To receive all Objects that are published or are received after this
subscription is processed, a subscriber can use an AbsoluteStart
filter with Start = {0, 0}.
AbsoluteRange (0x4): The filer Start Location and End Group are
specified explicitly in the SUBSCRIBE message. The Start specified
in the SUBSCRIBE message MAY be less than the Largest Object observed
at the publisher. If the specified End Group is the same group
specified in Start, the remainder of that Group passes the filter.
End Group MUST specify the same or a larger Group than specified in
Start.
A filter type other than the above MUST be treated as error.
Subscribe only delivers newly published or received Objects. Objects
from the past are retrieved using FETCH (Section 8.13).
A Subscription can also request a publisher to not forward Objects
for a given track by setting the Forward field to 0. This allows the
publisher or relay to prepare to serve the subscription in advance,
reducing the time to receive objects in the future. Relays SHOULD
set the Forward flag to 1 if a new subscription needs to be sent
upstream, regardless of the value of the Forward field from the
downstream subscription. Subscriptions that are not forwarded
consume resources from the publisher, so a publisher might
deprioritize, reject, or close those subscriptions to ensure other
subscriptions can be delivered. Control messages, such as
SUBCRIBE_DONE (Section 8.12) are still sent.
The format of SUBSCRIBE is as follows:
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SUBSCRIBE Message {
Type (i) = 0x3,
Length (i),
Request ID (i),
Track Alias (i),
Track Namespace (tuple),
Track Name Length (i),
Track Name (..),
Subscriber Priority (8),
Group Order (8),
Forward (8),
Filter Type (i),
[Start Location (Location)],
[End Group (i)],
Number of Parameters (i),
Subscribe Parameters (..) ...
}
Figure 9: MOQT SUBSCRIBE Message
* Request ID: See Section 8.1.
* Track Alias: A session specific identifier for the track. Data
streams and datagrams specify the Track Alias instead of the Track
Name and Track Namespace to reduce overhead. If the Track Alias
is already being used for a different track, the publisher MUST
close the session with a Duplicate Track Alias error
(Section 3.4).
* Track Namespace: Identifies the namespace of the track as defined
in (Section 2.4.1).
* Track Name: Identifies the track name as defined in
(Section 2.4.1).
* Subscriber Priority: Specifies the priority of a subscription
relative to other subscriptions in the same session. Lower
numbers get higher priority. See Section 6.
* Group Order: Allows the subscriber to request Objects be delivered
in Ascending (0x1) or Descending (0x2) order by group. See
Section 6. A value of 0x0 indicates the original publisher's
Group Order SHOULD be used. Values larger than 0x2 are a protocol
error.
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* Forward: If 1, Objects matching the subscription are forwarded to
the subscriber. If 0, Objects are not forwarded to the
subscriber. Any other value is a protocol error and MUST
terminate the session with a Protocol Violation (Section 3.4).
* Filter Type: Identifies the type of filter, which also indicates
whether the Start and End Group fields will be present.
* Start Location: The starting location for this subscriptions.
Only present for "AbsoluteStart" and "AbsoluteRange" filter types.
* End Group: The end Group ID, inclusive. Only present for the
"AbsoluteRange" filter type.
* Subscribe Parameters: The parameters are defined in Section 8.2.1.
On successful subscription, the publisher MUST reply with a
SUBSCRIBE_OK, allowing the subscriber to determine the start group/
object when not explicitly specified and the publisher SHOULD start
delivering objects.
If a publisher cannot satisfy the requested start or end or if the
end has already been published it SHOULD send a SUBSCRIBE_ERROR with
code 'Invalid Range'. A publisher MUST NOT send objects from outside
the requested start and end.
8.8. SUBSCRIBE_OK
A publisher sends a SUBSCRIBE_OK control message for successful
subscriptions.
SUBSCRIBE_OK Message {
Type (i) = 0x4,
Length (i),
Request ID (i),
Expires (i),
Group Order (8),
Content Exists (8),
[Largest Location (Location)],
Number of Parameters (i),
Subscribe Parameters (..) ...
}
Figure 10: MOQT SUBSCRIBE_OK Message
* Request ID: The Request ID of the SUBSCRIBE this message is
replying to Section 8.7.
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* Expires: Time in milliseconds after which the subscription is no
longer valid. A value of 0 indicates that the subscription does
not expire or expires at an unknown time. Expires is advisory and
a subscription can end prior to the expiry time or last longer.
* Group Order: Indicates the subscription will be delivered in
Ascending (0x1) or Descending (0x2) order by group. See
Section 6. Values of 0x0 and those larger than 0x2 are a protocol
error.
* Content Exists: 1 if an object has been published on this track, 0
if not. If 0, then the Largest Group ID and Largest Object ID
fields will not be present. Any other value is a protocol error
and MUST terminate the session with a Protocol Violation
(Section 3.4).
* Largest Location: The location of the largest object available for
this track. This field is only present if Content Exists has a
value of 1.
* Subscribe Parameters: The parameters are defined in Section 8.2.1.
8.9. SUBSCRIBE_ERROR
A publisher sends a SUBSCRIBE_ERROR control message in response to a
failed SUBSCRIBE.
SUBSCRIBE_ERROR Message {
Type (i) = 0x5,
Length (i),
Request ID (i),
Error Code (i),
Error Reason (Reason Phrase),
Track Alias (i),
}
Figure 11: MOQT SUBSCRIBE_ERROR Message
* Request ID: The Request ID of the SUBSCRIBE this message is
replying to Section 8.7.
* Error Code: Identifies an integer error code for subscription
failure.
* Error Reason: Provides the reason for subscription error. See
Section 1.3.3.
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* Track Alias: When Error Code is 'Retry Track Alias', the
subscriber SHOULD re-issue the SUBSCRIBE with this Track Alias
instead. If this Track Alias is already in use, the subscriber
MUST close the connection with a Duplicate Track Alias error
(Section 3.4).
The application SHOULD use a relevant error code in SUBSCRIBE_ERROR,
as defined below:
+======+==========================+
| Code | Reason |
+======+==========================+
| 0x0 | Internal Error |
+------+--------------------------+
| 0x1 | Unauthorized |
+------+--------------------------+
| 0x2 | Timeout |
+------+--------------------------+
| 0x3 | Not Supported |
+------+--------------------------+
| 0x4 | Track Does Not Exist |
+------+--------------------------+
| 0x5 | Invalid Range |
+------+--------------------------+
| 0x6 | Retry Track Alias |
+------+--------------------------+
| 0x10 | Malformed Auth Token |
+------+--------------------------+
| 0x11 | Unknown Auth Token Alias |
+------+--------------------------+
| 0x12 | Expired Auth Token |
+------+--------------------------+
Table 4
* Internal Error - An implementation specific or generic error
occurred.
* Unauthorized - The subscriber is not authorized to subscribe to
the given track.
* Timeout - The subscription could not be completed before an
implementation specific timeout. For example, a relay could not
establish an upstream subscription within the timeout.
* Not Supported - The endpoint does not support the SUBSCRIBE
method.
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* Track Does Not Exist - The requested track is not available at the
publisher.
* Invalid Range - The end of the SUBSCRIBE range is earlier than the
beginning, or the end of the range has already been published.
* Retry Track Alias - The publisher requires the subscriber to use
the given Track Alias when subscribing.
* Malformed Auth Token - Invalid Auth Token serialization during
registration (see Section 8.2.1.1).
* Unknown Auth Token Alias - Authorization Token refers to an alias
that is not registered (see Section 8.2.1.1).
* Expired Auth Token - Authorization token has expired
Section 8.2.1.1).
8.10. SUBSCRIBE_UPDATE
A subscriber issues a SUBSCRIBE_UPDATE to a publisher to request a
change to an existing subscription. Subscriptions can only become
more narrow, not wider, because an attempt to widen a subscription
could fail. If Objects before the start or after the end of the
current subscription are needed, a fetch might be able to retrieve
objects before the start. The start Object MUST NOT decrease and
when it increases, there is no guarantee that a publisher will not
have already sent Objects before the new start Object. The end Group
MUST NOT increase and when it decreases, there is no guarantee that a
publisher will not have already sent Objects after the new end
Object. A publisher SHOULD close the Session as a 'Protocol
Violation' if the SUBSCRIBE_UPDATE violates either rule or if the
subscriber specifies a Request ID that has not existed within the
Session.
There is no control message in response to a SUBSCRIBE_UPDATE,
because it is expected that it will always succeed and the worst
outcome is that it is not processed promptly and some extra objects
from the existing subscription are delivered.
Unlike a new subscription, SUBSCRIBE_UPDATE can not cause an Object
to be delivered multiple times. Like SUBSCRIBE, End Group MUST be
greater than or equal to the Group specified in Start.
If a parameter included in SUBSCRIBE is not present in
SUBSCRIBE_UPDATE, its value remains unchanged. There is no mechanism
to remove a parameter from a subscription.
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The format of SUBSCRIBE_UPDATE is as follows:
SUBSCRIBE_UPDATE Message {
Type (i) = 0x2,
Length (i),
Request ID (i),
Start Location (Location),
End Group (i),
Subscriber Priority (8),
Forward (8),
Number of Parameters (i),
Subscribe Parameters (..) ...
}
Figure 12: MOQT SUBSCRIBE_UPDATE Message
* Request ID: The Request ID of the SUBSCRIBE (Section 8.7) this
message is updating. This MUST match an existing Request ID.
* Start Location : The starting location.
* End Group: The end Group ID, plus 1. A value of 0 means the
subscription is open-ended.
* Subscriber Priority: Specifies the priority of a subscription
relative to other subscriptions in the same session. Lower
numbers get higher priority. See Section 6.
* Forward: If 1, Objects matching the subscription are forwarded to
the subscriber. If 0, Objects are not forwarded to the
subscriber. Any other value is a protocol error and MUST
terminate the session with a Protocol Violation (Section 3.4).
* Subscribe Parameters: The parameters are defined in Section 8.2.1.
8.11. UNSUBSCRIBE
A subscriber issues a UNSUBSCRIBE message to a publisher indicating
it is no longer interested in receiving media for the specified track
and requesting that the publisher stop sending Objects as soon as
possible.
The format of UNSUBSCRIBE is as follows:
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UNSUBSCRIBE Message {
Type (i) = 0xA,
Length (i),
Request ID (i)
}
Figure 13: MOQT UNSUBSCRIBE Message
* Request ID: The Request ID of the subscription that is being
terminated. See Section 8.7.
8.12. SUBSCRIBE_DONE
A publisher sends a SUBSCRIBE_DONE message to indicate it is done
publishing Objects for that subscription. The Status Code indicates
why the subscription ended, and whether it was an error. Because
SUBSCRIBE_DONE is sent on the control stream, it is likely to arrive
at the receiver before late-arriving objects, and often even late-
opening streams. However, the receiver uses it as an indication that
it should receive any late-opening streams in a relatively short
time.
Note that some objects in the subscribed track might never be
delivered, because a stream was reset, or never opened in the first
place, due to the delivery timeout.
A sender MUST NOT send SUBSCRIBE_DONE until it has closed all streams
it will ever open, and has no further datagrams to send, for a
subscription. After sending SUBSCRIBE_DONE, the sender can
immediately destroy subscription state, although stream state can
persist until delivery completes. The sender might persist
subscription state to enforce the delivery timeout by resetting
streams on which it has already sent FIN, only deleting it when all
such streams have received ACK of the FIN.
A sender MUST NOT destroy subscription state until it sends
SUBSCRIBE_DONE, though it can choose to stop sending objects (and
thus send SUBSCRIBE_DONE) for any reason.
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A subscriber that receives SUBSCRIBE_DONE SHOULD set a timer of at
least its delivery timeout in case some objects are still inbound due
to prioritization or packet loss. The subscriber MAY dispense with a
timer if it sent UNSUBSCRIBE or is otherwise no longer interested in
objects from the track. Once the timer has expired, the receiver
destroys subscription state once all open streams for the
subscription have closed. A subscriber MAY discard subscription
state earlier, at the cost of potentially not delivering some late
objects to the application. The subscriber SHOULD send STOP_SENDING
on all streams related to the subscription when it deletes
subscription state.
The format of SUBSCRIBE_DONE is as follows:
SUBSCRIBE_DONE Message {
Type (i) = 0xB,
Length (i),
Request ID (i),
Status Code (i),
Stream Count (i),
Error Reason (Reason Phrase)
}
Figure 14: MOQT SUBSCRIBE_DONE Message
* Request ID: The Request ID of the subscription that is being
terminated. See Section 8.7.
* Status Code: An integer status code indicating why the
subscription ended.
* Stream Count: An integer indicating the number of data streams the
publisher opened for this subscription. This helps the subscriber
know if it has received all of the data published in this
subscription by comparing the number of streams received. The
subscriber can immediately remove all subscription state once the
same number of streams have been processed. If the track had
Forwarding Preference = Datagram, the publisher MUST set Stream
Count to 0. If the publisher is unable to set Stream Count to the
exact number of streams opened for the subscription, it MUST set
Stream Count to 2^62 - 1. Subscribers SHOULD use a timeout or
other mechanism to remove subscription state in case the publisher
set an incorrect value, reset a stream before the SUBGROUP_HEADER,
or set the maximum value. If a subscriber receives more streams
for a subscription than specified in Stream Count, it MAY close
the session with a Protocol Violation.
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* Error Reason: Provides the reason for subscription error. See
Section 1.3.3.
The application SHOULD use a relevant status code in SUBSCRIBE_DONE,
as defined below:
+======+====================+
| Code | Reason |
+======+====================+
| 0x0 | Internal Error |
+------+--------------------+
| 0x1 | Unauthorized |
+------+--------------------+
| 0x2 | Track Ended |
+------+--------------------+
| 0x3 | Subscription Ended |
+------+--------------------+
| 0x4 | Going Away |
+------+--------------------+
| 0x5 | Expired |
+------+--------------------+
| 0x6 | Too Far Behind |
+------+--------------------+
Table 5
* Internal Error - An implementation specific or generic error
occurred.
* Unauthorized - The subscriber is no longer authorized to subscribe
to the given track.
* Track Ended - The track is no longer being published.
* Subscription Ended - The publisher reached the end of an
associated Subscribe filter.
* Going Away - The subscriber or publisher issued a GOAWAY message.
* Expired - The publisher reached the timeout specified in
SUBSCRIBE_OK.
* Too Far Behind - The publisher's queue of objects to be sent to
the given subscriber exceeds its implementation defined limit.
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8.13. FETCH
A subscriber issues a FETCH to a publisher to request a range of
already published objects within a track. The publisher responding
to a FETCH is responsible for delivering all available Objects in the
requested range in the requested order. The Objects in the response
are delivered on a single unidirectional stream. Any gaps in the
Group and Object IDs in the response stream indicate objects that do
not exist (eg: they implicitly have status Object Does Not Exist).
For Ascending Group Order this includes ranges between the first
requested object and the first object in the stream; between objects
in the stream; and between the last object in the stream and the
Largest Group/Object indicated in FETCH_OK, so long as the fetch
stream is terminated by a FIN. If no Objects exist in the requested
range, the publisher returns FETCH_ERROR with code No Objects.
*Fetch Types*
There are three types of Fetch messages:
Standalone Fetch (0x1) : A Fetch of Objects performed independently
of any Subscribe.
Relative Joining Fetch (0x2) : A Fetch joined together with a
Subscribe by specifying the Request ID of an active subscription and
a relative starting offset. A publisher receiving a Joining Fetch
uses properties of the associated Subscribe to determine the Track
Namespace, Track, Start Group, Start Object, End Group, and End
Object such that it is contiguous with the associated Subscribe. The
Joining Fetch begins the Preceding Group Offset prior to the
associated subscription.
Absolute Joining Fetch (0x3) : Identical to a Relative Joining Fetch
except that the Start Group is determined by an absolute Group value
rather than a relative offset to the subscription.
A Subscriber can use a Joining Fetch to, for example, fill a playback
buffer with a certain number of groups prior to the live edge of a
track.
A Joining Fetch is only permitted when the associated Subscribe has
the Filter Type Latest Object.
A Fetch Type other than 0x1, 0x2 or 0x3 MUST be treated as an error.
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A publisher responds to a FETCH request with either a FETCH_OK or a
FETCH_ERROR message. The publisher creates a new unidirectional
stream that is used to send the Objects. The FETCH_OK or FETCH_ERROR
can come at any time relative to object delivery.
A relay that has cached objects from the beginning of the range MAY
start sending objects immediately in response to a FETCH. If it
encounters an object in the requested range that is not cached and
has unknown status, the relay MUST pause subsequent delivery until it
has confirmed the object's status upstream. If the upstream FETCH
fails, the relay sends a FETCH_ERROR and can reset the unidirectional
stream. It can choose to do so immediately or wait until the cached
objects have been delivered before resetting the stream.
The Object Forwarding Preference does not apply to fetches.
Fetch specifies an inclusive range of Objects starting at Start
Object in Start Group and ending at End Object in End Group. End
Group and End Object MUST specify the same or a larger Location than
Start Group and Start Object.
The format of FETCH is as follows:
FETCH Message {
Type (i) = 0x16,
Length (i),
Request ID (i),
Subscriber Priority (8),
Group Order (8),
Fetch Type (i),
[Track Namespace (tuple),
Track Name Length (i),
Track Name (..),
Start Group (i),
Start Object (i),
End Group (i),
End Object (i),]
[Joining Subscribe ID (i),
Joining Start (i),]
Number of Parameters (i),
Parameters (..) ...
}
Figure 15: MOQT FETCH Message
Fields common to all Fetch Types:
* Request ID: See Section 8.1.
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* Subscriber Priority: Specifies the priority of a fetch request
relative to other subscriptions or fetches in the same session.
Lower numbers get higher priority. See Section 6.
* Group Order: Allows the subscriber to request Objects be delivered
in Ascending (0x1) or Descending (0x2) order by group. See
Section 6. A value of 0x0 indicates the original publisher's
Group Order SHOULD be used. Values larger than 0x2 are a protocol
error.
* Fetch Type: Identifies the type of Fetch, whether Standalone,
Relative Joining or Absolute Joining.
* Parameters: The parameters are defined in Section 8.2.1.
Fields present only for Standalone Fetch (0x1):
* Track Namespace: Identifies the namespace of the track as defined
in (Section 2.4.1).
* Track Name: Identifies the track name as defined in
(Section 2.4.1).
* Start Group: The start Group ID.
* Start Object: The start Object ID.
* End Group: The end Group ID.
* End Object: The end Object ID, plus 1. A value of 0 means the
entire group is requested.
Fields present only for Relative Fetch (0x2) and Absolute Fetch
(0x3):
* Joining Subscribe ID: The Request ID of the existing subscription
to be joined. If a publisher receives a Joining Fetch with a
Request ID that does not correspond to an existing Subscribe in
the same session, it MUST respond with a Fetch Error with code
Invalid Joining Subscribe ID.
* Joining Start : for a Relative Joining Fetch (0x2), this value
represents the group offset for the Fetch prior and relative to
the Current Group of the corresponding Subscribe. A value of 0
indicates the Fetch starts at the beginning of the Current Group.
For an Absolute Joining Fetch (0x3), this value represents the
Starting Group ID.
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Objects that are not yet published will not be retrieved by a FETCH.
The latest available Object is indicated in the FETCH_OK, and is the
last Object a fetch will return if the End Group and End Object have
not yet been published.
A publisher MUST send fetched groups in the determined group order,
either ascending or descending. Within each group, objects are sent
in Object ID order; subgroup ID is not used for ordering.
If Start Group/Start Object is greater than the latest published
Object group, the publisher MUST return FETCH_ERROR with error code
'Invalid Range'.
8.13.1. Calculating the Range of a Relative Joining Fetch
A publisher that receives a Fetch of type Type 0x2 treats it as a
Fetch with a range dynamically determined by the Preceding Group
Offset and field values derived from the corresponding subscription.
The Largest Group ID and Largest Object ID values from the
corresponding subscription are used to calculate the end of a
Relative Joining Fetch so the Objects retrieved by the FETCH and
SUBSCRIBE are contiguous and non-overlapping. If no Objects have
been published for the track, and the SUBSCRIBE_OK has a Content
Exists value of 0, the publisher MUST respond with a FETCH_ERROR with
error code 'Invalid Range'.
The publisher receiving a Relative Joining Fetch computes the range
as follows:
* Fetch Start Group: Subscribe Largest Group - Joining start
* Fetch Start Object: 0
* Fetch End Group: Subscribe Largest Group
* Fetch End Object: Subscribe Largest Object
A Fetch End Object of 0 requests the entire group, but Fetch will not
retrieve Objects that have not yet been published, so 1 is subtracted
from the Fetch End Group if Fetch End Object is 0.
8.13.2. Calculating the Range of an Absolute Joining Fetch
Identical to the Relative Joining fetch except that Fetch Start Group
is the Joining Start value.
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8.14. FETCH_OK
A publisher sends a FETCH_OK control message in response to
successful fetches. A publisher MAY send Objects in response to a
FETCH before the FETCH_OK message is sent, but the FETCH_OK MUST NOT
be sent until the end group and object are known.
FETCH_OK Message {
Type (i) = 0x18,
Length (i),
Request ID (i),
Group Order (8),
End Of Track (8),
End Location (Location),
Number of Parameters (i),
Subscribe Parameters (..) ...
}
Figure 16: MOQT FETCH_OK Message
* Request ID: The Request ID of the FETCH this message is replying
to Section 8.7.
* Group Order: Indicates the fetch will be delivered in Ascending
(0x1) or Descending (0x2) order by group. See Section 6. Values
of 0x0 and those larger than 0x2 are a protocol error.
* End Of Track: 1 if all objects have been published on this track,
so the End Group ID and Object Id indicate the last Object in the
track, 0 if not.
* End Location: The largest object covered by the FETCH response.
This is the minimum of the {End Group,End Object} specified in
FETCH and the largest known {group,object}. If the relay is
currently subscribed to the track, the largest known
{group,object} at the relay is used. For tracks with a requested
end larger than what is cached without an active subscription, the
relay makes an upstream request in order to satisfy the FETCH.
* Subscribe Parameters: The parameters are defined in Section 8.2.1.
8.15. FETCH_ERROR
A publisher sends a FETCH_ERROR control message in response to a
failed FETCH.
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FETCH_ERROR Message {
Type (i) = 0x19,
Length (i),
Request ID (i),
Error Code (i),
Error Reason (Reason Phrase)
}
Figure 17: MOQT FETCH_ERROR Message
* Request ID: The Request ID of the FETCH this message is replying
to Section 8.7.
* Error Code: Identifies an integer error code for fetch failure.
* Error Reason: Provides the reason for fetch error. See
Section 1.3.3.
The application SHOULD use a relevant error code in FETCH_ERROR, as
defined below:
+======+==============================+
| Code | Reason |
+======+==============================+
| 0x0 | Internal Error |
+------+------------------------------+
| 0x1 | Unauthorized |
+------+------------------------------+
| 0x2 | Timeout |
+------+------------------------------+
| 0x3 | Not Supported |
+------+------------------------------+
| 0x4 | Track Does Not Exist |
+------+------------------------------+
| 0x5 | Invalid Range |
+------+------------------------------+
| 0x6 | No Objects |
+------+------------------------------+
| 0x7 | Invalid Joining Subscribe ID |
+------+------------------------------+
| 0x10 | Malformed Auth Token |
+------+------------------------------+
| 0x11 | Unknown Auth Token Alias |
+------+------------------------------+
| 0x12 | Expired Auth Token |
+------+------------------------------+
Table 6
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* Internal Error - An implementation specific or generic error
occurred.
* Unauthorized - The subscriber is not authorized to fetch from the
given track.
* Timeout - The fetch could not be completed before an
implementation specific timeout. For example, a relay could not
FETCH missing objects within the timeout.
* Not supported - The endpoint does not support the FETCH method.
* Track Does Not Exist - The requested track is not available at the
publisher.
* Invalid Range - The end of the requested range is earlier than the
beginning, the start of the requested range is beyond the Largest
Object, or the track has not published any Objects yet.
* No Objects - No Objects exist between the requested Start and End
Locations.
* Invalid Joining Subscribe ID - The joining Fetch referenced a
Request ID that did not belong to an active Subscription.
* Malformed Auth Token - Invalid Auth Token serialization during
registration (see Section 8.2.1.1).
* Unknown Auth Token Alias - Authorization Token refers to an alias
that is not registered (see Section 8.2.1.1).
* Expired Auth Token - Authorization token has expired
Section 8.2.1.1).
8.16. FETCH_CANCEL
A subscriber sends a FETCH_CANCEL message to a publisher to indicate
it is no longer interested in receiving objects for the fetch
identified by the 'Request ID'. The publisher SHOULD promptly close
the unidirectional stream, even if it is in the middle of delivering
an object.
The format of FETCH_CANCEL is as follows:
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FETCH_CANCEL Message {
Type (i) = 0x17,
Length (i),
Request ID (i)
}
Figure 18: MOQT FETCH_CANCEL Message
* Request ID: The Request ID of the FETCH (Section 8.13) this
message is cancelling.
8.17. TRACK_STATUS_REQUEST
A potential subscriber sends a 'TRACK_STATUS_REQUEST' message on the
control stream to obtain information about the current status of a
given track.
A TRACK_STATUS message MUST be sent in response to each
TRACK_STATUS_REQUEST.
TRACK_STATUS_REQUEST Message {
Type (i) = 0xD,
Length (i),
Request ID (i),
Track Namespace (tuple),
Track Name Length (i),
Track Name (..),
Number of Parameters (i),
Parameters (..) ...,
}
Figure 19: MOQT TRACK_STATUS_REQUEST Message
* Request ID: See Section 8.1.
* Track Namespace: Identifies the namespace of the track as defined
in (Section 2.4.1).
* Track Name: Identifies the track name as defined in
(Section 2.4.1).
* Parameters: The parameters are defined in Section 8.2.1.
8.18. TRACK_STATUS
A publisher sends a 'TRACK_STATUS' message on the control stream in
response to a TRACK_STATUS_REQUEST message.
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TRACK_STATUS Message {
Type (i) = 0xE,
Length (i),
Request ID (i),
Status Code (i),
Largest Location (Location),
Number of Parameters (i),
Parameters (..) ...,
}
Figure 20: MOQT TRACK_STATUS Message
* Request ID: The Request ID of the TRACK_STATUS_REQUEST this
message is replying to Section 8.18.
* Status Code: Provides additional information about the status of
the track. It MUST hold one of the following values. Any other
value is a malformed message.
0x00: The track is in progress, and subsequent fields contain the
highest group and object ID for that track.
0x01: The track does not exist. Subsequent fields MUST be zero, and
any other value is a malformed message.
0x02: The track has not yet begun. Subsequent fields MUST be zero.
Any other value is a malformed message.
0x03: The track has finished, so there is no "live edge." Subsequent
fields contain the highest Group and object ID known.
0x04: The publisher is a relay that cannot obtain the current track
status from upstream. Subsequent fields contain the largest group
and object ID known.
Any other value in the Status Code field is a malformed message.
TODO: Auth Failures
* Largest Location: represents the largest Object location observed
by the Publisher for an active subscription. If the publisher is
a relay without an active subscription, it SHOULD send a
TRACK_STATUS_REQUEST upstream or MAY subscribe to the track, to
obtain the same information. If neither is possible, it should
return the best available information with status code 0x04.
The Parameters are defined in Section 8.2.1.
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8.19. ANNOUNCE
The publisher sends the ANNOUNCE control message to advertise that it
has tracks available within the announced Track Namespace. The
receiver verifies the publisher is authorized to publish tracks under
this namespace.
ANNOUNCE Message {
Type (i) = 0x6,
Length (i),
Request ID (i),
Track Namespace (tuple),
Number of Parameters (i),
Parameters (..) ...,
}
Figure 21: MOQT ANNOUNCE Message
* Request ID: See Section 8.1.
* Track Namespace: Identifies a track's namespace as defined in
(Section 2.4.1)
* Parameters: The parameters are defined in Section 8.2.1.
8.20. ANNOUNCE_OK
The subscriber sends an ANNOUNCE_OK control message to acknowledge
the successful authorization and acceptance of an ANNOUNCE message.
ANNOUNCE_OK Message {
Type (i) = 0x7,
Length (i),
Request ID (i)
}
Figure 22: MOQT ANNOUNCE_OK Message
* Request ID: The Request ID of the ANNOUNCE this message is
replying to Section 8.19.
8.21. ANNOUNCE_ERROR
The subscriber sends an ANNOUNCE_ERROR control message for tracks
that failed authorization.
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ANNOUNCE_ERROR Message {
Type (i) = 0x8,
Length (i),
Request ID (i),
Error Code (i),
Error Reason (Reason Phrase)
}
Figure 23: MOQT ANNOUNCE_ERROR Message
* Request ID: The Request ID of the ANNOUNCE this message is
replying to Section 8.19.
* Error Code: Identifies an integer error code for announcement
failure.
* Error Reason: Provides the reason for announcement error. See
Section 1.3.3.
The application SHOULD use a relevant error code in ANNOUNCE_ERROR,
as defined below:
+======+==========================+
| Code | Reason |
+======+==========================+
| 0x0 | Internal Error |
+------+--------------------------+
| 0x1 | Unauthorized |
+------+--------------------------+
| 0x2 | Timeout |
+------+--------------------------+
| 0x3 | Not Supported |
+------+--------------------------+
| 0x4 | Uninterested |
+------+--------------------------+
| 0x10 | Malformed Auth Token |
+------+--------------------------+
| 0x11 | Unknown Auth Token Alias |
+------+--------------------------+
| 0x12 | Expired Auth Token |
+------+--------------------------+
Table 7
* Internal Error - An implementation specific or generic error
occurred.
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* Unauthorized - The subscriber is not authorized to announce the
given namespace.
* Timeout - The announce could not be completed before an
implementation specific timeout.
* Not Supported - The endpoint does not support the ANNOUNCE method.
* Uninterested - The namespace is not of interest to the endpoint.
* Malformed Auth Token - Invalid Auth Token serialization during
registration (see Section 8.2.1.1).
* Unknown Auth Token Alias - Authorization Token refers to an alias
that is not registered (see Section 8.2.1.1).
* Expired Auth Token - Authorization token has expired
Section 8.2.1.1).
8.22. UNANNOUNCE
The publisher sends the UNANNOUNCE control message to indicate its
intent to stop serving new subscriptions for tracks within the
provided Track Namespace.
UNANNOUNCE Message {
Type (i) = 0x9,
Length (i),
Track Namespace (tuple),
}
Figure 24: MOQT UNANNOUNCE Message
* Track Namespace: Identifies a track's namespace as defined in
(Section 2.4.1).
8.23. ANNOUNCE_CANCEL
The subscriber sends an ANNOUNCE_CANCEL control message to indicate
it will stop sending new subscriptions for tracks within the provided
Track Namespace.
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ANNOUNCE_CANCEL Message {
Type (i) = 0xC,
Length (i),
Track Namespace (tuple),
Error Code (i),
Error Reason (Reason Phrase),
}
Figure 25: MOQT ANNOUNCE_CANCEL Message
* Track Namespace: Identifies a track's namespace as defined in
(Section 2.4.1).
* Error Code: Identifies an integer error code for canceling the
announcement. ANNOUNCE_CANCEL uses the same error codes as
ANNOUNCE_ERROR (Section 8.21).
* Error Reason: Provides the reason for announcement cancelation.
See Section 1.3.3.
8.24. SUBSCRIBE_ANNOUNCES
The subscriber sends the SUBSCRIBE_ANNOUNCES control message to a
publisher to request the current set of matching announcements, as
well as future updates to the set.
SUBSCRIBE_ANNOUNCES Message {
Type (i) = 0x11,
Length (i),
Request ID (i),
Track Namespace Prefix (tuple),
Number of Parameters (i),
Parameters (..) ...,
}
Figure 26: MOQT SUBSCRIBE_ANNOUNCES Message
* Request ID: See Section 8.1.
* Track Namespace Prefix: An ordered N-Tuple of byte fields which
are matched against track namespaces known to the publisher. For
example, if the publisher is a relay that has received ANNOUNCE
messages for namespaces ("example.com", "meeting=123",
"participant=100") and ("example.com", "meeting=123",
"participant=200"), a SUBSCRIBE_ANNOUNCES for ("example.com",
"meeting=123") would match both. If an endpoint receives a Track
Namespace Prefix tuple with an N of 0 or more than 32, it MUST
close the session with a Protocol Violation.
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* Parameters: The parameters are defined in Section 8.2.1.
The publisher will respond with SUBSCRIBE_ANNOUNCES_OK or
SUBSCRIBE_ANNOUNCES_ERROR. If the SUBSCRIBE_ANNOUNCES is successful,
the publisher will forward any matching ANNOUNCE messages to the
subscriber that it has not yet sent. If the set of matching ANNOUNCE
messages changes, the publisher sends the corresponding ANNOUNCE or
UNANNOUNCE message.
A subscriber cannot make overlapping namespace subscriptions on a
single session. Within a session, if a publisher receives a
SUBSCRIBE_ANNOUNCES with a Track Namespace Prefix that is a prefix of
an earlier SUBSCRIBE_ANNOUNCES or vice versa, it MUST respond with
SUBSCRIBE_ANNOUNCES_ERROR, with error code Namespace Prefix Overlap.
The publisher MUST ensure the subscriber is authorized to perform
this namespace subscription.
SUBSCRIBE_ANNOUNCES is not required for a publisher to send ANNOUNCE
and UNANNOUNCE messages to a subscriber. It is useful in
applications or relays where subscribers are only interested in or
authorized to access a subset of available announcements.
8.25. SUBSCRIBE_ANNOUNCES_OK
A publisher sends a SUBSCRIBE_ANNOUNCES_OK control message for
successful namespace subscriptions.
SUBSCRIBE_ANNOUNCES_OK Message {
Type (i) = 0x12,
Length (i),
Request ID (i),
}
Figure 27: MOQT SUBSCRIBE_ANNOUNCES_OK Message
* Request ID: The Request ID of the SUBSCRIBE_ANNOUNCES this message
is replying to Section 8.24.
8.26. SUBSCRIBE_ANNOUNCES_ERROR
A publisher sends a SUBSCRIBE_ANNOUNCES_ERROR control message in
response to a failed SUBSCRIBE_ANNOUNCES.
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SUBSCRIBE_ANNOUNCES_ERROR Message {
Type (i) = 0x13,
Length (i),
Request ID (i),
Error Code (i),
Error Reason (Reason Phrase)
}
Figure 28: MOQT SUBSCRIBE_ANNOUNCES_ERROR Message
* Request ID: The Request ID of the SUBSCRIBE_ANNOUNCES this message
is replying to Section 8.24.
* Error Code: Identifies an integer error code for the namespace
subscription failure.
* Error Reason: Provides the reason for the namespace subscription
error. See Section 1.3.3.
The application SHOULD use a relevant error code in
SUBSCRIBE_ANNOUNCES_ERROR, as defined below:
+======+==========================+
| Code | Reason |
+======+==========================+
| 0x0 | Internal Error |
+------+--------------------------+
| 0x1 | Unauthorized |
+------+--------------------------+
| 0x2 | Timeout |
+------+--------------------------+
| 0x3 | Not Supported |
+------+--------------------------+
| 0x4 | Namespace Prefix Unknown |
+------+--------------------------+
| 0x5 | Namespace Prefix Overlap |
+------+--------------------------+
| 0x10 | Malformed Auth Token |
+------+--------------------------+
| 0x11 | Unknown Auth Token Alias |
+------+--------------------------+
| 0x12 | Expired Auth Token |
+------+--------------------------+
Table 8
* Internal Error - An implementation specific or generic error
occurred.
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* Unauthorized - The subscriber is not authorized to subscribe to
the given namespace prefix.
* Timeout - The operation could not be completed before an
implementation specific timeout.
* Not Supported - The endpoint does not support the
SUBSCRIBE_ANNOUNCES method.
* Namespace Prefix Unknown - The namespace prefix is not available
for subscription.
* Namespace Prefix Overlap - The namespace prefix overlaps with
another SUBSCRIBE_ANNOUNCES in the same session.
* Malformed Auth Token - Invalid Auth Token serialization during
registration (see Section 8.2.1.1).
* Unknown Auth Token Alias - Authorization Token refers to an alias
that is not registered (see Section 8.2.1.1).
* Expired Auth Token - Authorization token has expired
Section 8.2.1.1).
8.27. UNSUBSCRIBE_ANNOUNCES
A subscriber issues a UNSUBSCRIBE_ANNOUNCES message to a publisher
indicating it is no longer interested in ANNOUNCE and UNANNOUNCE
messages for the specified track namespace prefix.
The format of UNSUBSCRIBE_ANNOUNCES is as follows:
UNSUBSCRIBE_ANNOUNCES Message {
Type (i) = 0x14,
Length (i),
Track Namespace Prefix (tuple)
}
Figure 29: MOQT UNSUBSCRIBE Message
* Track Namespace Prefix: As defined in Section 8.24.
9. Data Streams and Datagrams
A publisher sends Objects matching a subscription on Data Streams or
Datagrams.
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All unidirectional MOQT streams start with a variable-length integer
indicating the type of the stream in question.
+===========+=================================+
| ID | Type |
+===========+=================================+
| 0x08-0x0D | SUBGROUP_HEADER (Section 9.4.2) |
+-----------+---------------------------------+
| 0x05 | FETCH_HEADER (Section 9.4.4) |
+-----------+---------------------------------+
Table 9
All MOQT datagrams start with a variable-length integer indicating
the type of the datagram.
+===========+======================================+
| ID | Type |
+===========+======================================+
| 0x00-0x01 | OBJECT_DATAGRAM (Section 9.2) |
+-----------+--------------------------------------+
| 0x02-0x03 | OBJECT_DATAGRAM_STATUS (Section 9.2) |
+-----------+--------------------------------------+
Table 10
An endpoint that receives an unknown stream or datagram type MUST
close the session.
The publisher only sends Objects after receiving a SUBSCRIBE or
FETCH. The publisher MUST NOT send Objects that are not requested.
If an endpoint receives an Object it never requested, it SHOULD
terminate the session with a protocol violation. Objects can arrive
after a subscription or fetch has been cancelled, so the session MUST
NOT be teriminated in that case.
Every Track has a single 'Object Forwarding Preference' and the
Original Publisher MUST NOT mix different forwarding preferences
within a single track. If a subscriber receives Objects via both
Subgroup streams and Datagrams in response to a SUBSCRIBE, it SHOULD
close the session with an error of 'Protocol Violation'
9.1. Objects
An Object contains a range of contiguous bytes from the specified
track, as well as associated metadata required to deliver, cache, and
forward it. Objects are sent by publishers.
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9.1.1. Canonical Object Properties
A canonical MoQ Object has the following information:
* Track Namespace and Track Name: The track this object belongs to.
* Group ID: The object is a member of the indicated group ID
Section 2.3 within the track.
* Object ID: The order of the object within the group.
* Publisher Priority: An 8 bit integer indicating the publisher's
priority for the Object Section 6.
* Object Forwarding Preference: An enumeration indicating how a
publisher sends an object. The preferences are Subgroup and
Datagram. When in response to a SUBSCRIBE, an Object MUST be sent
according to its Object Forwarding Preference, described below.
* Subgroup ID: The object is a member of the indicated subgroup ID
(Section 2.2) within the group. This field is omitted if the
Object Forwarding Preference is Datagram.
* Object Status: As enumeration used to indicate missing objects or
mark the end of a group or track. See Section 9.1.1.1 below.
* Object Extension Length: The total length of the Object Extension
Headers block, in bytes.
* Object Extensions : A sequence of Object Extension Headers. See
Section 9.1.1.2 below.
* Object Payload: An opaque payload intended for an End Subscriber
and SHOULD NOT be processed by a relay. Only present when 'Object
Status' is Normal (0x0).
9.1.1.1. Object Status
The Object Status informs subscribers what objects will not be
received because they were never produced, are no longer available,
or because they are beyond the end of a group or track.
Status can have following values:
* 0x0 := Normal object. This status is implicit for any non-zero
length object. Zero-length objects explicitly encode the Normal
status.
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* 0x1 := Indicates Object does not exist. Indicates that this
object does not exist at any publisher and it will not be
published in the future. This SHOULD be cached.
* 0x3 := Indicates end of Group. ObjectId is one greater that the
largest object produced in the group identified by the GroupID.
This is sent right after the last object in the group. If the
ObjectID is 0, it indicates there are no Objects in this Group.
This SHOULD be cached. A publisher MAY use an end of Group object
to signal the end of all open Subgroups in a Group.
* 0x4 := Indicates end of Track. GroupID is either the largest
group produced in this track and the ObjectID is one greater than
the largest object produced in that group, or GroupID is one
greater than the largest group produced in this track and the
ObjectID is zero. This status also indicates the last group has
ended. An object with this status that has a Group ID less than
any other GroupID, or an ObjectID less than or equal to the
largest in the specified group, is a protocol error, and the
receiver MUST terminate the session. This SHOULD be cached.
Any other value SHOULD be treated as a protocol error and terminate
the session with a Protocol Violation (Section 3.4). Any object with
a status code other than zero MUST have an empty payload.
9.1.1.2. Object Extension Header
Any Object may have extension headers except those with Object Status
'Object Does Not Exist'. If an endpoint receives a non-existent
Object containing extension headers it MUST close the session with a
Protocol Violation.
Object Extension Headers are visible to relays and allow the
transmission of future metadata relevant to MOQT Object distribution.
Any Object metadata never accessed by the transport or relays SHOULD
be serialized as part of the Object payload and not as an extension
header.
Extension Headers are defined in external specifications and
registered in an IANA table Section 12. These specifications define
the type and value of the header, along with any rules concerning
processing, modification, caching and forwarding. A relay which is
coded to implement these rules is said to "support" the extension.
If unsupported by the relay, Extension Headers MUST NOT be modified,
MUST be cached as part of the Object and MUST be forwarded by relays.
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If supported by the relay and subject to the processing rules
specified in the definition of the extension, Extension Headers MAY
be modified, added, removed, and/or cached by relays.
Object Extension Headers are serialized as Key-Value-Pairs Figure 2.
Header types are registered in the IANA table 'MOQ Extension
Headers'. See Section 12.
9.2. Object Datagram
An OBJECT_DATAGRAM carries a single object in a datagram.
An Object received in an OBJECT_DATAGRAM message has an Object
Forwarding Preference = Datagram. To send an Object with Object
Forwarding Preference = Datagram, determine the length of the header
and payload and send the Object as datagram. In certain scenarios
where the object size can be larger than maximum datagram size for
the session, the Object will be dropped.
OBJECT_DATAGRAM {
Type (i),
Track Alias (i),
Group ID (i),
Object ID (i),
Publisher Priority (8),
[Extension Headers Length (i),
Extension headers (...)],
Object Payload (..),
}
Figure 30: MOQT OBJECT_DATAGRAM
The Type field takes the form 0b0000000X (or the set of values from
0x00 to 0x01). The LSB of the type determines if the Extensions
Headers Length and Extension headers are present. If an endpoint
receives a datagram with Type 0x01 and Extension Headers Length is 0,
it MUST close the session with Protocol Violation.
There is no explicit length field. The entirety of the transport
datagram following Publisher Priority contains the Object Payload.
9.3. Object Datagram Status
An OBJECT_DATAGRAM_STATUS is similar to OBJECT_DATAGRAM except it
conveys an Object Status and has no payload.
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OBJECT_DATAGRAM_STATUS {
Type (i),
Track Alias (i),
Group ID (i),
Object ID (i),
Publisher Priority (8),
[Extension Headers Length (i),
Extension headers (...)],
Object Status (i),
}
Figure 31: MOQT OBJECT_DATAGRAM_STATUS
The Type field takes the form 0b0000001X (or the set of values from
0x02 to 0x03). The LSB of the type determines if the Extensions
Headers Length and Extension headers are present. If an endpoint
receives a datagram with Type 0x03 and Extension Headers Length is 0,
it MUST close the session with Protocol Violation.
9.4. Streams
When objects are sent on streams, the stream begins with a Subgroup
Header and is followed by one or more sets of serialized object
fields. If a stream ends gracefully in the middle of a serialized
Object, the session SHOULD be terminated with a Protocol Violation.
A publisher SHOULD NOT open more than one stream at a time with the
same Subgroup Header field values.
9.4.1. Stream Cancellation
Streams aside from the control stream MAY be canceled due to
congestion or other reasons by either the publisher or subscriber.
Early termination of a stream does not affect the MoQ application
state, and therefore has no effect on outstanding subscriptions.
9.4.2. Subgroup Header
When a stream begins with SUBGROUP_HEADER, all Objects on the stream
belong to the track requested in the Subscribe message identified by
Track Alias and the subgroup indicated by 'Group ID' and Subgroup ID.
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SUBGROUP_HEADER {
Type (i),
Track Alias (i),
Group ID (i),
[Subgroup ID (i),]
Publisher Priority (8),
}
Figure 32: MOQT SUBGROUP_HEADER
All Objects received on a stream opened with SUBGROUP_HEADER have an
Object Forwarding Preference = Subgroup.
There are 6 defined Type values for SUBGROUP_HEADER:
+======+===============+=================+============+
| Type | Subgroup ID | Subgroup ID | Extensions |
+======+===============+=================+============+
| | Field Present | Value | Present |
+------+---------------+-----------------+------------+
| 0x08 | No | 0 | No |
+------+---------------+-----------------+------------+
| 0x09 | No | 0 | Yes |
+------+---------------+-----------------+------------+
| 0x0A | No | First Object ID | No |
+------+---------------+-----------------+------------+
| 0x0B | No | First Object ID | Yes |
+------+---------------+-----------------+------------+
| 0x0C | Yes | N/A | No |
+------+---------------+-----------------+------------+
| 0x0D | Yes | N/A | Yes |
+------+---------------+-----------------+------------+
Table 11
For Type values where Subgroup ID Field Present is No, there is no
explicit Subgroup ID field in the header and the Subgroup ID is
either 0 (for Types 0x08-09) or the Object ID of the first object
transmitted in this subgroup (for Types 0x0A-0B).
For Type values where Extensions Present is No, Extensions Headers
Length is not present and all Objects have no extensions. When
Extensions Present is Yes, Extension Headers Length is present in all
Objects in this subgroup. Objects with no extensions set Extension
Headers Length to 0.
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To send an Object with Object Forwarding Preference = Subgroup, find
the open stream that is associated with the subscription, Group ID
and Subgroup ID, or open a new one and send the SUBGROUP_HEADER.
Then serialize the following fields.
The Object Status field is only sent if the Object Payload Length is
zero.
{
Object ID (i),
[Extension Headers Length (i),
Extension headers (...)],
Object Payload Length (i),
[Object Status (i)],
Object Payload (..),
}
Figure 33: MOQT Subgroup Object Fields
A publisher MUST NOT send an Object on a stream if its Object ID is
less than a previously sent Object ID within a given group in that
stream.
9.4.3. Closing Subgroup Streams
Subscribers will often need to know if they have received all objects
in a Subgroup, particularly if they serve as a relay or cache. QUIC
and Webtransport streams provide signals that can be used for this
purpose. Closing Subgroups promptly frees system resources and often
unlocks flow control credit to open more streams.
If a sender has delivered all objects in a Subgroup to the QUIC
stream, except any objects before the beginning of a subscription, it
MUST close the stream with a FIN.
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If a sender closes the stream before delivering all such objects to
the QUIC stream, it MUST use a RESET_STREAM or RESET_STREAM_AT
[I-D.draft-ietf-quic-reliable-stream-reset] frame. This includes an
open Subgroup exceeding its Delivery Timeout, early termination of
subscription due to an UNSUBSCRIBE message, a publisher's decision to
end the subscription early, or a SUBSCRIBE_UPDATE moving the end of
the subscription to before the current Group or the start after the
current Group. When RESET_STREAM_AT is used, the reliable_size
SHOULD include the stream header so the receiver can identify the
corresponding subscription and accurately account for reset data
streams when handling SUBSCRIBE_DONE (see Section 8.12). Publishers
that reset data streams without using RESET_STREAM_AT with an
appropriate reliable_size can cause subscribers to hold on to
subscription state until a timeout expires.
A sender might send all objects in a Subgroup and the FIN on a QUIC
stream, and then reset the stream. In this case, the receiving
application would receive the FIN if and only if all objects were
received. If the application receives all data on the stream and the
FIN, it can ignore any RESET_STREAM it receives.
If a sender will not deliver any objects from a Subgroup, it MAY send
a SUBGROUP_HEADER on a new stream, with no objects, and then send
RESET_STREAM_AT with a reliable_size equal to the length of the
stream header. This explicitly tells the receiver there is an unsent
Subgroup.
A relay MUST NOT forward an Object on an existing Subgroup stream
unless it is the next Object in that Subgroup. A relay knows that an
Object is the next Object in the Subgroup if at least one of the
following is true: * the Object ID is one greater than the previous
Object sent on this Subgroup stream. * the Object was received on
the same upstream Subgroup stream as the previously sent Object on
the downstream Subgroup stream, with no other Objects in between. *
it knows all Object IDs between the current and previous Object IDs
on the Subgroup stream belong to different Subgroups or do not exist.
If the relay does not know if an Object is the next Object, it MUST
reset the Subgroup stream and open a new one to forward it.
Since SUBSCRIBEs always end on a group boundary, an ending
subscription can always cleanly close all its subgroups. A sender
that terminates a stream early for any other reason (e.g., to handoff
to a different sender) MUST use RESET_STREAM or RESET_STREAM_AT.
Senders SHOULD terminate a stream on Group boundaries to avoid doing
so.
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An MoQT implementation that processes a stream FIN is assured it has
received all objects in a subgroup from the start of the
subscription. If a relay, it can forward stream FINs to its own
subscribers once those objects have been sent. A relay MAY treat
receipt of EndOfGroup, GroupDoesNotExist, or EndOfTrack objects as a
signal to close corresponding streams even if the FIN has not
arrived, as further objects on the stream would be a protocol
violation.
Similarly, an EndOfGroup message indicates the maximum Object ID in
the Group, so if all Objects in the Group have been received, a FIN
can be sent on any stream where the entire subgroup has been sent.
This might be complex to implement.
Processing a RESET_STREAM or RESET_STREAM_AT means that there might
be other objects in the Subgroup beyond the last one received. A
relay might immediately reset the corresponding downstream stream, or
it might attempt to recover the missing Objects in an effort send all
the objects in the subgroups and the FIN. It also might send
RESET_STREAM_AT with reliable_size set to the last object it has, so
as to reliably deliver the objects it has while signaling that other
objects might exist.
A subscriber MAY send a QUIC STOP_SENDING frame for a subgroup stream
if the Group or Subgroup is no longer of interest to it. The
publisher SHOULD respond with RESET_STREAM or RESET_STREAM_AT. If
RESET_STREAM_AT is sent, note that the receiver has indicated no
interest in the objects, so setting a reliable_size beyond the stream
header is of questionable utility.
RESET_STREAM and STOP_SENDING on SUBSCRIBE data streams have no
impact on other Subgroups in the Group or the subscription, although
applications might cancel all Subgroups in a Group at once.
The application SHOULD use a relevant error code in RESET_STREAM or
RESET_STREAM_AT, as defined below:
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+======+==================+
| Code | Reason |
+======+==================+
| 0x0 | Internal Error |
+------+------------------+
| 0x1 | Cancelled |
+------+------------------+
| 0x2 | Delivery Timeout |
+------+------------------+
| 0x3 | Session Closed |
+------+------------------+
Table 12
Internal Error: An implementation specific error
Cancelled: The subscriber requested cancellation via UNSUBSCRIBE,
FETCH_CANCEL or STOP_SENDING, or the publisher ended the
subscription, in which case SUBSCRIBE_DONE (Section 8.12) will
have a more detailed status code.
Delivery Timeout: The DELIVERY TIMEOUT Section 8.2.1.2 was exceeded
for this stream
Session Closed: The publisher session is being closed
9.4.4. Fetch Header
When a stream begins with FETCH_HEADER, all objects on the stream
belong to the track requested in the Fetch message identified by
Request ID.
FETCH_HEADER {
Request ID (i),
}
Figure 34: MOQT FETCH_HEADER
Each object sent on a fetch stream after the FETCH_HEADER has the
following format:
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{
Group ID (i),
Subgroup ID (i),
Object ID (i),
Publisher Priority (8),
Extension Headers Length (i),
[Extension headers (...)],
Object Payload Length (i),
[Object Status (i)],
Object Payload (..),
}
Figure 35: MOQT Fetch Object Fields
The Object Status field is only sent if the Object Payload Length is
zero.
The Subgroup ID field of an object with a Forwarding Preference of
"Datagram" (see Section 9.1.1) is set to the Object ID.
9.5. Examples
Sending a subgroup on one stream:
Stream = 2
SUBGROUP_HEADER {
Type = 0
Track Alias = 2
Group ID = 0
Subgroup ID = 0
Publisher Priority = 0
}
{
Object ID = 0
Object Payload Length = 4
Payload = "abcd"
}
{
Object ID = 1
Object Payload Length = 4
Payload = "efgh"
}
Sending a group on one stream, with the first object containing two
Extension Headers.
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Stream = 2
SUBGROUP_HEADER {
Type = 1
Track Alias = 2
Group ID = 0
Publisher Priority = 0
}
{
Object ID = 0
Extension Headers Length = 33
{ Type = 4
Value = 2186796243
},
{ Type = 77
Length = 21
Value = "traceID:123456"
}
Object Payload Length = 4
Payload = "abcd"
}
{
Object ID = 1
Extension Headers Length = 0
Object Payload Length = 4
Payload = "efgh"
}
10. Extension Headers
The following Object Extension Headers are defined in MoQT.
10.1. Prior Group ID Gap
Prior Group ID Gap (Extension Header Type 0x40) is a variable length
integer containing the number of Groups prior to the current Group
that do not and will never exist. For example, if the Original
Publisher published an Object in Group 7 and knows it will never
publish any Objects in Group 8 or Group 9, it can include Prior Group
ID Gap = 2 in any number of Objects in Group 10, as it sees fit. A
track with a Group that contains more than one Object with different
values for Prior Group ID Gap or has a Prior Group ID Gap larger than
the Group ID is considered malformed. If an endpoint receives an
Object with a Group ID within a previously communicated gap it also
treats the track as malformed.
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This extension is optional, as publishers might not know the prior
gap gize, or there may not be a gap. If Prior Group ID Gap is not
present, the receiver cannot infer any information about the
existence of prior groups (see Section 2.3.1).
This extension can be added by the Original Publisher, but MUST NOT
be added by relays. This extension MUST NOT be modified or removed.
11. Security Considerations
TODO: Expand this section, including subscriptions.
11.1. Resource Exhaustion
Live content requires significant bandwidth and resources. Failure
to set limits will quickly cause resource exhaustion.
MOQT uses stream limits and flow control to impose resource limits at
the network layer. Endpoints SHOULD set flow control limits based on
the anticipated bitrate.
Endpoints MAY impose a MAX STREAM count limit which would restrict
the number of concurrent streams which an application could have in
flight.
The publisher prioritizes and transmits streams out of order.
Streams might be starved indefinitely during congestion. The
publisher and subscriber MUST cancel a stream, preferably the lowest
priority, after reaching a resource limit.
11.2. Timeouts
Implementations are advised to use timeouts to prevent resource
exhaustion attacks by a peer that does not send expected data within
an expected time. Each implementation is expected to set its own
limits.
12. IANA Considerations
TODO: fill out currently missing registries:
* MOQT version numbers
* Setup parameters
* Subscribe parameters
* Subscribe Error codes
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* Subscribe Namespace Error codes
* Announce Error codes
* Announce Cancel Reason codes
* Message types
* MOQ Extension headers - we wish to reserve extension types 0-63
for standards utilization where space is a premium, 64 - 16383 for
standards utilization where space is less of a concern, and 16384
and above for first-come-first-served non-standardization usage.
* MOQT Auth Token Type
TODO: register the URI scheme and the ALPN and grease the Extension
types
Contributors
The original design behind this protocol was inspired by three
independent proposals: WARP [I-D.draft-lcurley-warp] by Luke Curley,
RUSH [I-D.draft-kpugin-rush] by Kirill Pugin, Nitin Garg, Alan
Frindell, Jordi Cenzano and Jake Weissman, and QUICR
[I-D.draft-jennings-moq-quicr-proto] by Cullen Jennings, Suhas
Nandakumar and Christian Huitema. The authors of those documents
merged their proposals to create the first draft of moq-transport.
The IETF MoQ Working Group received an enormous amount of support
from many people. The following people provided substantive
contributions to this document:
* Ali Begen
* Charles Krasic
* Christian Huitema
* Cullen Jennings
* James Hurley
* Jordi Cenzano
* Kirill Pugin
* Luke Curley
* Martin Duke
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* Mike English
* Mo Zanaty
* Will Law
References
Normative References
[I-D.draft-ietf-quic-reliable-stream-reset]
Seemann, M. and K. Oku, "QUIC Stream Resets with Partial
Delivery", Work in Progress, Internet-Draft, draft-ietf-
quic-reliable-stream-reset-06, 28 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
reliable-stream-reset-06>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/rfc/rfc8615>.
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[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[WebTransport]
Frindell, A., Kinnear, E., and V. Vasiliev, "WebTransport
over HTTP/3", Work in Progress, Internet-Draft, draft-
ietf-webtrans-http3-12, 3 March 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-
webtrans-http3-12>.
Informative References
[I-D.draft-jennings-moq-quicr-proto]
Jennings, C. F., Nandakumar, S., and C. Huitema, "QuicR -
Media Delivery Protocol over QUIC", Work in Progress,
Internet-Draft, draft-jennings-moq-quicr-proto-01, 11 July
2022, <https://datatracker.ietf.org/doc/html/draft-
jennings-moq-quicr-proto-01>.
[I-D.draft-kpugin-rush]
Pugin, K., Garg, N., Frindell, A., Ferret, J. C., and J.
Weissman, "RUSH - Reliable (unreliable) streaming
protocol", Work in Progress, Internet-Draft, draft-kpugin-
rush-03, 21 April 2025,
<https://datatracker.ietf.org/doc/html/draft-kpugin-rush-
03>.
[I-D.draft-lcurley-warp]
Curley, L., Pugin, K., Nandakumar, S., and V. Vasiliev,
"Warp - Live Media Transport over QUIC", Work in Progress,
Internet-Draft, draft-lcurley-warp-04, 13 March 2023,
<https://datatracker.ietf.org/doc/html/draft-lcurley-warp-
04>.
[I-D.ietf-webtrans-overview]
Vasiliev, V., "The WebTransport Protocol Framework", Work
in Progress, Internet-Draft, draft-ietf-webtrans-overview-
09, 25 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-
webtrans-overview-09>.
[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, DOI 10.17487/RFC6582, April 2012,
<https://www.rfc-editor.org/rfc/rfc6582>.
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[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[RFC9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
"CUBIC for Fast and Long-Distance Networks", RFC 9438,
DOI 10.17487/RFC9438, August 2023,
<https://www.rfc-editor.org/rfc/rfc9438>.
Appendix A. Change Log
RFC Editor's Note: Please remove this section prior to publication of
a final version of this document.
Issue and pull request numbers are listed with a leading octothorp.
A.1. Since draft-ietf-moq-transport-10
* Added Common Structure definitions - Location, Key-Value-Pair and
Reason Phrase
* Limit lengths of all variable length fields, including Track
Namespace and Name
* Control Message length is now 16 bits instead of variable length
* Subscribe ID became Request ID, and was added to most control
messages. Request ID is used to correlate OK/ERROR responses for
ANNOUNCE, SUBSCRIBE_ANNOUNCES, and TRACK_STATUS. Like Subscribe
ID, Request IDs are flow controlled.
* Explain rules for caching in more detail
* Changed the SETUP parameter format for even number parameters to
match the Object Header Extension format
* Rotated SETUP code points
* Added Parameters to TRACK_STATUS and TRACK_STATUS_REQUEST
* Clarified how subscribe filters work
* Added Next Group Filter to SUBSCRIBE
* Added Forward flag to SUBSCRIBE
* Renamed FETCH_OK field to End and clarified how to set it
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* Added Absolute Joining Fetch
* Clarified No Error vs Invalid Range FETCH_ERROR cases
* Use bits in SUBGROUP_HEADER and DATAGRAM* types to compress
subgroup ID and extensions
* Coalesced END_OF_GROUP and END_OF_TRACK_AND_GROUP status
* Objects that Do Not Exist cannot have extensions when sent on the
wire
* Specified error codes for resetting data streams
* Defined an Object Header Extension for communicating a known Group
ID gap
* Replaced AUTHORIZATION_INFO with AUTHORIZATION_TOKEN, which has
more structure, compression, and additional Auth related error
codes (#760)
Authors' Addresses
Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
Victor Vasiliev
Google
Email: vasilvv@google.com
Ian Swett (editor)
Google
Email: ianswett@google.com
Alan Frindell (editor)
Meta
Email: afrind@meta.com
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